The loveliest explanation of AUM is found within the ancient Vedic and Sanskrit traditions. We can read about AUM in the marvellous Manduka Upanishad, which explains the four elements of AUM as an allegory of the four planes of consciousness.
“A” resonates in the centre of the mouth. It represents normal waking consciousness, in which subject and object exist as separate entities. This is the level of mechanics, science, logical reason, the lower three chakras. Matter exists on a gross level, is stable and slow to change.
Then the sound “U” transfers the sense of vibration to the back of the mouth, and shifts the allegory to the level of dream consciousness. Here, object and subject become intertwined in awareness. Both are contained within us. Mallei becomes subtle, more fluid, rapidly changing. This is the realm of dreams, divinities, imagination, the inner world.
“M” is the third element, humming with lips gendy closed. This sound resonates forward in the mouth and buzzes throughout the head. (Try it.) This sound represents the realm of deep, dreamless sleep. There is neither observing subject nor observed object. All are one, and nothing. Only pure Consciousness exists – unseen, pristine, latent, covered with darkness. This is the cosmic night, the interval between cycles of Creation – the womb of the Divine Mother.
It might be said that the ultimate aim of Yoga is to enter this third dreamless realm while awake. Yoga means “yoke” or “join”. Through Yoga we “join” our waking consciousness to its “source” in the world of pure, quality-less Consciousness.
Which brings us to the fourth sound of AUM, the primal “unstruck” sound within the silence at the end of the sacred syllable. In fact, the word “silence” itself can be understood only in reference to “sound”. We hear this silence best when listening to sound, any sound at all, without interpreting or judging the sound.
Listening fully, openly, without preconceptions or expectations. The sound of music, the sound of the city, the sound of the wind in the forest. All can give us the opportunity to follow the path of sound into the awareness of the sound behind the sound.