Below is some material from Shiv Brat Lal, Maharshi Mehi, Swami Santsevi Ji, and Swami Vyasanand about inner Sound Meditation, useful points from rare writings of Sant Mat that some may benefit from.
"Various sorts of Sound Currents reverberate in the human system from which the initiate has to pick up the right one and listen to it, otherwise he will go astray and lose his equipoise. The practical Guru forewarns his disciple and directs which Sound to listen to and which one to discard." (Shiv Brat Lal)
"...Only the Central Sound has the power to attract the consciousness to the center and carry the soul to the center of a higher realm. Other illusory material sounds do not have that magnetism to attract the consciousness to the higher realms." (Swami Vyasanand)
The Masters teach that the world -- indeed many realms -- are illusion, and that on inner planes there are many reflections or copies of higher realities. There are phenomena than can imitate or mimic spiritual experiences. There are even realms that to some extent might appear to be heavens or copies of Sach Khand. Hall of Mirrors. Matrix. Labyrinth. Astral fly-paper. In this world there is the illusion of the master, a meat-eating guru offering instruction and initiation, but also who says ethics do not matter -- the illusion of having a spiritual teacher. The illusion of satsang. The illusion of enlightenment. One can stare at a bright light or contemplate the flame of a candle, then close one's eyes in a darkened room and continue to see a kind of photo-like retinal image of that candle -- an optical illusion of inner light. Some electronic devices and electrical appliances generate white noise, humming sounds, or high frequency ringing tones and overtones. These to some extent can mimic inner spiritual Sounds -- the illusion of the Sound Current.
Sometimes in the news there have been reports about people hearing a mysterious sound that seems to be coming from nowhere. One of these has been called "the Taos Hum." In, Philosophy of Liberation, Maharshi Mehi states that not only are there spiritual sounds associated with the heavenly realms, but there are also many vibrations or semi-subtle sounds associated with the gross-material realm. He says one should not falsely assume they are accessing the real inner Sound Current just by hearing ANY kind of sound that may manifest itself in one's perception during the silence of meditation: "There are sounds due to gross vibrations in the material body. To meditate on these gross-material sounds and believe it to be as the full practice of the Yoga of Sound shows a lack of knowledge of Yoga." There are gross-material sounds one can hear in the silence of meditation that do not lead one Above, do not lead to spiritual growth, transformation or enlightenment. Swami Vyasanand:
"This is because only the Central Sound has the power to attract the consciousness to the center and carry the soul to the center of a higher realm. Other illusory material sounds do not have that magnetism to attract the consciousness to the higher realms."
There are inner Sounds that one receives instruction from a true Master about at the time of initiation into Surat Shabd Yoga (Inner Sound Meditation, the Yoga of the Audible Life Stream). These Sounds are perceived to come from the right side or center and represent the Ascending Current. Sounds that appear to be coming from the left side are of the descending current flowing downward into the material creation or multiverse. One is instructed to focus on the Sounds of the Ascending Current or Audible Life Stream -- certain Sounds. There are of course many other sounds one might hear, but one has to focus on certain Sound Currents. The reason for this is these Sounds, if focused upon during meditation practice, will keep pulling the soul’s attention up to ever higher and higher levels. These are coming from regions Above. Other sounds, though mistaken for the Sound Current, are interesting, miraculous or even mystical, will not help the Surat-soul to ascend or make spiritual progress.
"Various sorts of Sound Currents reverberate in the human system from which the initiate has to pick up the right one and listen to it, otherwise he will go astray and lose his equipoise. The practical Guru forewarns his disciple and directs which Sound to listen to and which one to discard." (Shiv Brat Lal)
"I have heard accounts of some practitioners who follow unusual sound meditation practices and claim to hear sounds. However, these gross-material sounds are imaginary or generated by metabolic functions and blood circulation. An ear disease can also create a humming sound that can be mistaken for the inner Divine Sound." (Swami Vyasanand)
Shiv Brat Lal: "The devotee in his spiritual journey upwards hears sweet melodies which attract him. The melodies are a powerful magnetic force which draws the attention inwards and makes it fully attuned to proceed up and up."
Maharshi Mehi in, Philosophy of Liberation: "Each of the Sounds of the centers of the five realms possesses an attribute to draw Surat (the consciousness) upward to its center. The Essential Sound or Pure Spiritual Sound has the attribute to draw one’s consciousness up to the Supreme Sovereign [God]. The other mayavi sounds (material Sounds mentioned above) of each of the respective five realms have the attribute to attract the person who hears it (or focuses on it) and to draw that person’s Surat (consciousness) to the Sound of the realm directly above. Further, for the practitioner, the realization of the Essential Divine Sound is impossible without first attaining the Sounds of the lower realms. Sar Shabda (the Essential Divine Sound) beckons a return to the Supreme Sovereign. The progression of Sound in all centers of the realms is upward, carrying one’s consciousness from the lower centers to the higher centers ultimately reaching the Supreme Sovereign God." -- Philosophy Of Liberation: https://spiritualawakeningradio.com/Philosophy_Of_Liberation_Sant_Mat.pdf
In, The Inward Journey of the Soul (Chal Hansa Nij Desh), Swami Vyasanand says, "The question arises: What is the secret of correct technique for Shabd (Sound) meditation? Sant Radhaswami says: 'Focus your consciousness stream on the Inner Space. You will find the Divine Sound which will lead you beyond the snares of this illusory world.' You should focus your consciousness or inner Current in the middle of the eyebrows -- the meeting point of the Ida and Pingla channels -- which is known as the tenth gate, Sushumna, Ajna Chakra or third eye. The practitioner will attain one-pointedness through this practice. In other words, the subtlest and most luminous sign of the light realm (bindu) will appear. With that luminous sign the inner Sound will also manifest.
"Sant Paltu Sahab says; 'Out from that burning light comes the Divine Sound, and only the one immersed in the Samadhi (meditation) of Divine knowledge can hear, none other.' This is the Sound that is heard when concentrating on that first mark of light. Due to carelessness, it is possible to also hear the other sounds of the lower realms that have the potential to distract the consciousness Current. It is conceivable then, that the upward ascent of consciousness becomes distracted, thus creating a hindrance in progress. This is because only the Central Sound has the power to attract the consciousness to the center and carry the soul to the center of a higher realm. Other illusory material sounds do not have that magnetism to attract the consciousness to the higher realms.
-- Swami Vyasanand Ji Maharaj
More from Swami Vyasanand: Sant Kabir Sahab says: "A rare practitioner who abides by the knowledge given by the Guru listens to the subtle Sound of Inner Space. First that Sound emanates from the bindu or Ajna Chakra (third eye). A practitioner who can withdraw his mind from the nine gates of the body and concentrate in the tenth gate, he or she alone is able to hear that subtle Sound."
Sant Tulsi Sahab says: "Enter into the Inner empty Space (Shunya) of Sushumna and listen to the Sound that is beckoning you toward the Divine. This Sound has been vibrating since the beginning of creation. If you want to meet your True Friend (Supreme Being) then constantly and carefully listen to that Sound. This body is the sacred holy place (Ka'ba, the holiest place in Islam) created by the Divine, and within it is found the arch (the higher realms of macrocosm beginning from the tenth gate). It is the inner sanctum within, in which is found the Divine Sound reverberating. O practitioner, listen with full concentration; the Sound that originated from the Divine is constantly reverberating and calling you."
Someone must have asked, generally, if the Sound is not heard with the physical ears then with what kind of ear is this subtle Sound heard? When the mind becomes concentrated, then the physical sense of hearing becomes quiet and one does not hear. This occurs because the attention withdraws from the senses and one is unable to hear physical sound.
In response Sant Tulsi Sahab says: "A practitioner who is able to focus in the Sushumna or the tenth gate for some time will experience the opening of the inner subtle hearing faculty. That inner sense of hearing is also known as the consciousness Current." My Guru Maharshi Mehi says: "A practitioner hears the sweet Sound of the flute through the stream of inner consciousness."
When the outer ear ceases to hear and the mind is fully concentrated within, then the inner hearing opens up and the practitioner can hear the Divine Sound within (through the inner ear). From this, do not infer that a deaf person could easily hear that inner Sound, because complete concentration is the key, not a lack of physical hearing. Sants have articulated about the closing of three gates of the body to experience the inner Sound. The three gates are eyes, ears, and mouth. Close these and, only then, can one hear the inner Sounds. According to Sant Kabir: "Close your eyes, ears, and mouth, and listen to the Anahad (inner Unstruck Divine Sound)." Sant Nanak Sahab says: "Close the three gates and listen to the reverberation of the Divine Sound." Sant Maharshi Mehi and also Sufi Sants have used similar vocabulary to describe the process of closing three openings for hearing the inner Sound.
I have heard accounts of some practitioners who follow unusual sound meditation practices and claim to hear sounds. However, these gross-material sounds are imaginary or generated by metabolic functions and blood circulation. An ear disease can also create a humming sound that can be mistaken for the inner Divine Sound. Some false gurus boast about their imaginary inner experiences of Divine Light and Sound in order to impress their followers.
Other Hatha Yogis perform postures (asanas) such as mula bandh (contracting the muscles of lower organs), uddiyana bandha (contracting the abdominal muscles and diaphragm that involves breathing exercises), and jalandhara bandha (throat lock to the flow of energy in the nerves and blood flow in the neck). Some practitioners use ketchari mudra (touching the root of the palate with the tongue) and attempt sound meditation in that mudra. Still others try to listen from the right or from the left ear. Some listen from below 'the tenth gate' and others above 'the tenth gate.' Some press their eardrums with their fingers and hear vibrations and think them to be the Divine Sound. Some stare at the tip of the nose; some listen to music in the dark, and some in the light. Some place their head down, others straighten their chest. In today's world, in the name of the Divine Sound there is a prevalence of a great variety of illusory and imaginary practices claiming to be techniques of Sound meditation. These are not only against the teachings of Sants but are also incapable of providing any real inner experience of Divine Sound.
Sant Tulsi Sahab declares: "Without the true knowledge of the Yoga of Sound, all efforts are hopeless. Only a Satguru who is accomplished can impart this knowledge about the different forms of Sound." Therefore, Maharshi Mehi says, "Practicing Sound Yoga (Inner Sound Meditation) is not child’s play." Sant Kabir says: "Without practice of this Sound meditation man is subjected to birth and death." Therefore having learned from an accomplished teacher, the practitioner would gain the ability to hear the Celestial, not illusory, sounds.
Closing of the Ears in Meditation
The closing of the eyes, ears, and mouth is used to help tune out the distractions caused by the outer senses. This is essential for the purpose of focusing one's gaze in the Sushumna. Therefore, in the initial stages it is advised that the practitioner close these three senses of sight, hearing, and speaking. Consequently, the mind begins to be focused in the third eye. For those whose consciousness stream has become mature in the power of concentration, it is not necessary to close these three sensory organs. This is due to the fact that the practitioner's mind is focused on the target of the 'third eye' becomes detached from the senses and is no longer held by experiences arising from the sensory objects.
The secret of practicing Sound Yoga is to attach the consciousness to the center of the 'third eye.' It is necessary for the following reasons: First, by not following this instruction, the practitioner will be distracted by the lower physical sounds imbued by rajas guna (passion) and tamas guna (dark inertia). The consciousness Current would not be able to grasp the Nada (Essential Central Sound) and ascend to the higher realms. It gives the practitioner only a satisfaction of listening to a sound -- but not the true benefit of the experience of the Sar Shabd (the Essential Sound)...
-- Swami Vyasanand, The Inward Journey of the Soul (Chal Hansa Nij Desh)
Swami Santsevi Ji Maharaj in, "Quintessence of Yoga, The Secrets of Success", writes: "Closing the ears when we hear the Divine Sound within ourselves is the best approach. There is no harm in it. We can include this along with other practices. This practice of inner Sound can go on in the night, because there is no noise at that time and environment is peaceful...When the inner Divine Sound merges into the imperishable Supreme Lord, there is the absolute State of Soundlessness (the Ultimate Reality)."
You may ask, what does it mean to "close the ears"? There is a technique initiates learn to do just that in order to focus on inner Sound. Also, sometimes people use earplugs to block outer sounds during inner Sound practice [bhajan]. But there is another meaning. Some clarification is given in the book, "Harmony of All Religions", by Santsevi Ji, who states that closing the ears is primarily an internal reality that occurs after one's awareness [attention] has gone within: "When you focus your gaze within the ajna chakra [third eye] then automatically your physical ears will close and the inner ears will open."
Kirpal Singh: "Once the life-stream becomes audible, one is never lonely; for he hears its reverberations at home and abroad. The Voice of God keeps reminding him of the true home of his Father. The practice of Sound Principle rids one of all troubles and afflictions..." 
