This is a beautifully written paper that explains the four levels of Vak (speech) achieved through audible and inaudible sound. The practice is meant to raise consciousness. It involves pranayama, internal alchemy kundalini meditations, mantra japa, merging the mind with the mantra and awakening the inner ear to hear the celestial sounds.  This is Vag Yoga, a lineage of Tantric and Vedic science of the four levels of Vak, Speech