Thanks for the source. With anthaHstha-s it has been recorded by Panini too in his rule when they are in conjunction with a preceding "m":
हे मपरे वा । । ८,३.२६ । ।
नपरे नः । । ८,३.२७ । ।
मपरे हकारे परे मस्य मो वा. किम् ह्मलयति, किं ह्मलयति. (यवलपरे यवला वा). किंय्ह्यः, किं ह्यः. किंव्ह्वलयति, किं ह्वलयति. किंल् ह्लादयति, किं ह्लादयति etc.
In any case, in spite of phonological script Devanagari, not all the pronunciations could be represented orthographically. The conclusion would be better, even though we hear bramhma, or vahnhi, or so it may be a different in pronunciation that orthographically represented which is described as "aurasya". Or it may be written as we write now, and pronounced as such, which can be unknowingly "aurasya" even if we do not recognize it in association with the above letters noted in the ShikShA. Only that there is no separate orthographic representation than ह, like संवृत अकार and विवृत. In usage, it is संवृत as noted by the last rule by Panini, as pronounced by the Hindi Speakers, "अ अ इति। । ८,४.६८। । Like others are this is also विवृत and this last rule prescribes संवृत in usage. But no letter is assigned to it separately. Just like, औरस्य "h" doesn't have any special letter to note it. What we hear traditionally may be similar to "mhm" as in the above sequences, but not orthographically identical.
Whether it is due to the influence of Prakrit or regional languages via Prakrit is a different point to be discussed in the light of Historical linguistics and not in the traditional way that Veda-s are eternal words and the Vedanga-s including Paniniya, part of the eternal literature, in which there is no scope for historical discussion.
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Thanks for providing the source. Yet, the question is unresolved. It should be "h" pronounced as "aurasya" unlike other consonants. I could not find another consonant articulated among those listed and appropriated articulation in Siddhantakaumudi to compare with the pronunciation of any other consonant.
Thanks once again for bumping up the original thread of the discussion. It assimilates like "mhm" sequence may the tentative solution to the question raised long ago and now by Daniel.
We may be certain that it is not a case of metathesis.
--
Dr. Hari Narayana Bhat B.R.
EFEO,
PONDICHERRY