OM
“By pronouncing OM properly, the heart and the mind are filled with peace, tranquility and bliss. You can feel the limitless ocean of power and energy within you. In order to acquire and possess this invaluable treasure of Nature, the Bliss and Peace, OM works as a key to unlock the coffer. The sound of OM, filling the entire atmosphere, gives immediate relief to humanity. At the end of the prayers in Jewish, English or Arabian languages, the word Amen, or Aamin, is nothing but the changed form of OM. The last word of the Greek language is Omegh which has the prominence of OM. Also, in the English words, omnipresent, omnipotent and omniscient, the eminence of OM is distinct. The presence of OM in these words is not just by chance. It is on account of the word Om that these words give the indication of universalism. Om is like a huge tree, casting its cool shadow all around and providing peaceful and refreshing relief and energy to all those suffering from the scorching heat of pain, affliction, worry, anxiety, etc.
In every beginning of Koran, at the top, there are three letters, Alif (A ) Lam ( l ) and Mim (m ). Lam (L ) can be pronounced as Wao ( O ), according to Arabic grammar, under certain circumstances. In Arabic Shamsaldin is read as Samsuddin. Nizamaldin is read as Nizamuddin. According to Arabic grammar, when Lam (L ) comes between a vowel and a consonant, it becomes silent and gives the sound of Arabic Pesh (O or U ). Similar is the case here with Alif (A) , Lam (L ) and Mim (M). Here Lam (L ) is in between the vowel Alif (A ) and the consonant Mim (M) and, therefore, it becomes silent and gives the sound of O or U. Accordingly, Alif, Lam and Mim give the sound of Alif(A) Wao (O or U) and Mim(M) i.e. AOM oa AUM. This is nothing but Om. This is no secret of God, as it is said.
The conch shell, when blown, gives out the sound of Om which diverges in the space to spread peace, concentration and self-confidence in the atmosphere. This stimulates the heart to be merged in Godhood. It has been since birth. When a child is born, he cries with the sound of Om. When the children play and their minds are saturated with happiness, they produce a noise which resembles that of Om. When people fall sick or are afflicted with some unbearable pain, they invariably groan which produces the sound resembling Om. This gives them relief from their agonizing pain.” ~ Excerpt from the work of Rev. Swami Ramtirth.

“The vowel sounds produced by the voice are due to the vibrations of two cartilaginous plates, the "vocal chords" placed at the top of wind pipe, edge to edge with a narrow slit between them; air blown through this slit from the lungs keeps the plate vibrating. The apparatus is really a free reed, the vocal chords have muscles attached to them which can vary the frequency of the vibration and the pitch of the sound produced. The vowel are continuous sounds formed by varying the size and shape of mouth cavity. The consonants are particular interruptions of these sounds. They cut short the vowel sound but cannot themselves be sounded without vowels. And for this reason, the vowels are known as 'Shaktis'.” ~ Excerpt from Sir John Woodroffe

AUM contains three letters; A starts from the initial manifestation with throat, U comes to the mouth and M ceases with closing of lips. These cover the entire vocal phenomenon.....all words, languages are enshrined within these three...A,U,M.