The Cosmic Flute of Krishna -- Shabda-Nada in Ancient India
Millenia ago, Krishna playing his Bansuri flute as the cosmic musician, is one obvious image of the Celestial Song of God or "Bhagavad Gita", and we also find a clearly organized path of Inner Sound sadhana present in certain Upanishads many, many centuries before the time of Kabir, Mirabai, and other medieval Sants.
"Meditation on Nad or the Sound Principle
is the Royal Road to salvation."
-- Hansa Naad Upanishad
"Let the yogi sit on Sidh Asana and while practicing the Vaisnavi Mudra, should hear the sound through his right ear. By communion with the Word, he will become deaf to the external sounds, and will attain the Turya Pad or a state of equipoise within a fortnight. First the murmuring sounds resembling those of the waves of the ocean, the fall of rain and the running rivulets and the Bheri will be heard intermingled with the sounds of bell and conch,..."
-- Nad Bind Upanishad
