"This very Self, from the standpoint of the syllable, is OM. From the standpoint of the letters the quarters are the letters, and the letters are the quarters : a, u, m.
Vaishvanara, whose sphere is the waking state, is the first letter a, because of pervasiveness or being the first. He who knows thus, does verily attain all desirable things, and becomes the foremost.
Taijasa, whose sphere is the dream state, is the second letter u, because of excellence and intermediateness. How who knows thus increases the current of knowledge and becomes equal. None is born in his lineage who is not a knower of Brahman (the Supreme).
Prajna, whose sphere is the sleep state, is the third letter m, because of measuring or absorption. Anyone who knows thus measures all this, and becomes the place of absorption.
Turiya (fourth) is the partless OM – beyond empirical dealings, the culmination of phenomenal existence, the auspicious, and the non-dual. Indeed, Om is thus the Self. He who knows thus enters the Self through his self."
(Mandukya Upanishad, Verse 8-12)

English Audio CD - 72 min.