This post in another thread may be of use, too.
"Very interesting reading about AUM chanting. Would like to add some more information.
Basically, AUM is a primordial sound. “A” when the lips part to let the air in or out, “M” when the lips close again and “U” the whole trajectory in between. So simple, yet takes lifetime to really understand. The many benefits of AUM-chanting have already been mentioned in the posts.
Let us also know that one can begin there but can take AUM to much greater heights. Chanting it is relaxing and invigorating; listening to it is thousand times more so. AUM is a cosmic vibration that resonates in each cell, molecule and atom of our being. Listening to AUM from our entire being takes us in a meditative mode and at the feet of the inner guru. That’s why the association with the Ajna chakra.
Talking of higher octave or lower pitch to chant AUM, each has its effect on gross or subtle bodies. In Vedas, the sound frequency of extremely high vibrations of the fourth state (turiya) is called para-naad. That of the third, causal body, is 25% of it and called pashyanti-naad. That of the second, astral body, is 25% of pashyanti and called madhyama-naad and that of the first, physical body, is 25% of madhyama, and called vaikheri-naad. When we chant aloud, our whole range of sound is this vaikheri. We can only imagine what para-naad could be; but listening to AUM gives us some access to subtle levels. Yoga Masters say that when the inner guru starts teaching, AUM is the first topic."