The auditory Expression ‘AUM‘, as was the expression ‘Brahman‘, was a Mantric terms meant for Yagnic formalities before it took on conventional lingual meaning.
The Mantra ‘AUM‘ [the ‘Queen of Mantras’] is made-up of four elements. The beginning vowel ‘A’, the ending consonant ‘M’, and the middle ‘oo!’
They, between their stretch capture all of spoken sound and symbolically mark all ‘Expression’.
But the heart of the Mantra, the fourth element Turiya, is the unheard silence that complements the sonant expression itself.
Turiya translates literally as ‘The Immaculately Pure’. The original term for Shūnyam was as Pujyam: ‘That worthy of worship’. Turiya is the auditory analogue to Pūjyam.
And just as with ‘True Nothing’ getting calcified as ‘Being’, Turiya gets conceptualized and reified, assigned aspects and characteristics, by the religious-minded. Just as the symbol ‘0’ acquires ‘properties’ in the hands of the Mathematicians.
Soon, inscrutable connotations are imputed to ‘AUM’, secret formulas devised to reveal them and prayers rendered to access their powers. In time Turiya the Silence is declared the source of Sound itself and gets mystified into a perfect superstitious-laden opacity.
[This of course has a long history in religious literature, the ‘Creatio Ex-Nihilo‘ idea. The unburnt remains of an elevated modeled-understanding’.]