In May 2017, my teacher, Yoganand, send over this article that he found on the origin of OM!

The Origin of AUM
by Elst Koenraadelst – May 17, 2013
Read the complete article here.

Dirghatamas is believed to have been the court-priest of the early Vedic king Bharata. This king patronized the origin of the Vedic tradition. He was a descendent of Puru, hence his tribe is called Paurava, and the clan of which he was the ancestor, is called Bharata. The Mahabharata describes a fraternal fight within this royal clan. India itself is named Bharat after him. The name Dirghatamas, “long darkness”, may be a nickname chosen for its descriptive aptness: he was known as a star-gazer, and some of his astronomical findings are mentioned in the hymns attributed to him, Rg-Veda 1:140-164. He is also said to be the brother of Bharadwaj, known as the principal author of Rg-Vedic book 6 and leader of the earliest clan of seers, the Angiras.

His most famous hymn is Rg-Veda 1:164. Among the celebrated elements from it, most people will know the simile of the two birds, one eating and the other just looking on (later a parable for the ego and the Self); the first division of the circle in 12 and in 360; the concept of creation through sacrifice; and the much-quoted (and sometimes abused) phrase Ekam sad vipra bahudha vadanti, “truth is one, but the wise ones give it many names”. It is this hymn that also gave me the clue to the real origin of Aum.

On the one hand, verse 39 asks for the “syllable” of praise to the gods. The composer says it is a mystery, though known to the select people present. But the whole hymn talks of a sound not longer than a syllable.

(39 Upon what syllable of holy praise−song, as twere their highest heaven, the Gods repose them,−
Who knows not this, what will he do with praise−song? But they who know it well sit here assembled.)

On the other, in the preceding verses, the sound made by the cows is repeatedly mentioned, as well as the care of the cow for her young. The root  vat- means “year” (Latin vetus, “having years”, “old”), the word vatsa means “yearling”, “dependent child”, hence “calf”. What goes on between cow and calf is vatsalya, still the Hindi word for “tenderness”, “affection”. This affection is uttered by the cow’s lowing and the calf’s lowing back. Repeatedly, the cow is praised and the sound of the cow is invoked.

(26 I invocate the milch−cow good for milking so that the milker, deft of hand, may drain her.
May Savitar give goodliest stimulation. The caldron is made hot; I will proclaim it.
27 She, lady of all treasure, is come hither yearning in spirit for her calf and lowing.
May this cow yield her milk for both the Asvins, and may she prosper to our high advantage.
28 The cow hath lowed after her blinking youngling; she licks his forehead, as she lows, to form it.
His mouth she fondly calls to her warm udder, and suckles him with milk while gently lowing.)

So my penny dropped: the syllable that encompasses all Vedic hymns, that is also used in the beginning of the opening hymn, Aum, is nothing but a human vocalization of the sound made by the cow. In English it is usually rendered as Mooh.