It is the conceptual content of the spoken word that distinguishes 'speech' from 'sound'.
Speech as such could never 'return' sound to 'hearing'. For such 'return' only a meaningless word could suffice. That, no doubt, is the explanation of mantra, the 'meaning' of which is not only inessential but intentionally obscure.
The prajnaparamita mantram in the Hrdya Sutra is thus explicable as what it purports to be, i.e. the supreme and most direct vehicle of awakening. No doubt this applies also to Om Mani Padma Hum, and all the others.
But let us not confuse the presence or absence of conceptuality in the words of the mantram, and the understanding that we are 'returning' sound to hearing. Without that understanding neither a sonata nor the braying of a donkey should be likely to reveal the prajnatic factor: that understanding is fundamental in the sense of being the basis of the 'event', and the 'hearing', 'seeing', or other sensorial perception, is merely the medium.
We should not overlook the fact that the 'event' itself is phenomenal, a space-time performance, whereas the understanding, conceptually relative, non-conceptually does not appertain to split-mind but to its wholeness.