Some describe it as the same phonetic sound as "OM" produced when vocalized/externalized.

"Ah-mmmmmm"

"mmmmmmm"

It is the sound of silence. The sound of Mu. The sound of the void of space. The root of mind.

"Ancient teachings and modern science agree: you, I, all living things, all things in existence are made up at their most essential level of vibrating, pulsing energy.

For millennia, mystics have recounted their experience of this energy, which is said to manifest in our hearing awareness as a humming vibration around and within everything else.

In the Sanskrit tradition, this sound is called "Anahata Nada," the "Unstruck Sound."

...

And the ancients say that the audible sound which most resembles this unstruck sound is the syllable OM. Tradition has it that this ancient mantra is composed of four elements: the first three are vocal sounds: A, U, and M. The fourth sound, unheard, is the silence which begins and ends the audible sound, the silence which surrounds it." - Source: [link to www.spiritsound.com]