The mantra Om (or Aum) has been handed down to us by the Himalayan sages. It is the most important mantra of Yoga. According to tradition, every 'thing' manifest comes from Primordial Vibration, which is symbolised by Om: all material objects, all living beings, including each of us, all spiritual teachings, including Yoga, all languages, including Sanskrit, all scriptures, including the Vedas, everything.

Everything has come out of Primordial Vibration, which is represented by Om.

This concurs with modern scientific thinking which says that everything - every atom and molecule in every nook and corner of this universe - is formed out of energy vibration. Einstein formulated his famous equation that E = mc2 which indicates that matter (m) is but an expression of Energy (E). Every atom, at-Om, comes out of the Primordial Vibration which is symbolised by Om.

Om as a sound, syllable (Om or Aum) and glyph () all symbolise the fact that all material objects, all phenomena and all thought patterns, both on a microcosmic and macrocosmic level, are states of energy vibration.

From:  Mandala Yoga Ashram - Wales Uk

Pronounciation of Aum from Wikipedia

"A - emerges from the throat, originating in the region of the navel U - rolls over the tongue M - ends on the lips ; A - waking, U - dreaming, M - sleeping It is the sum and substance of all the words that can emanate from the human throat. It is the primordial fundamental sound symbolic of the Universal Absolute." [1].

In fact, when correctly pronounced, or rather, rendered, the "A" can be felt as a vibration that manifests itself near the navel or abdomen; the "U" can be felt vibrating the chest, and the "M" vibrates the cranium or the head. The abdominal vibration symbolises Creation; It is interesting that the "creative" or reproductive organs are also located in the lower abdomen. The vibration of the chest represents Preservation, which is also where the lungs are situated (the lungs sustain or preserve the body through breath). The vibration of the head is associated with Destruction or sacrifice, since all that one gives up or destroys is first destroyed mentally. Hence, the entire cycle of the universe and all it contains is said to be symbolised in AUM.

Today, in all Hindu art and all over India and Nepal, 'Aum' can be seen virtually everywhere, a standard sign for Hinduism and a vast but economical storehouse for the deep philosophy and mythology inherent in the world's oldest religion.

Notes the Chandogya Upanishad, "That syllable, is a syllable of permission; for, whenever we permit anything, we say Aum." However, this is seen by others as a myopic perspective because the same Hindu scriptures, the Upanishads, that aver this function also attribute to it the divine property of the source of the universe. Aum is seen as the source of existence as we know it within the causal dimensions of time and space, and thus affirmatory meanings in languages are a natural progression. Aum is not only affirmation, but negation, and transcends both.

The AUM sound is sometimes called "the 3-syllable Veda". The third syllable arises because in Devanagari and similar alphabets, a consonant at the end of a word is sometimes written as a separate consonant letter with the virama "no vowel" sign, and this combination is treated as a syllable when talking about Devanagari writing rather than about phonetics.

An individual's "Aum" is the sound that can be held steady the longest per breath for the longest consecutive sequence of breaths. It is called "aum" in every culture that is aware of it because it sounds like that in all humans. The lower pitches are more suited because they require less muscular contraction of the abdomen, leading to lower rates of oxygen consumption, allowing for longer time between breaths. The Aum is the exact sound that is easiest for the individual to produce.

Once the minimization of oxygen consumption occurs (by minimization of muscular exertion), the outflow of air will be steady and quite sensitive to any forces that alter the amount of pressure in the chest cavity. One of the most notable consequences of this is that the rhythmic contractions of the heart become audible within the Aum.

Thus, by the use of Aum:

• one can easily hear their own heart.
  
• a person can modify the pace of their heart.
  
• a group of people can synchronize their heartbeats.

From:  Wikipedia Definitions of Aum