What Witzel may be thinking about is the fact that in a sequence *mr- (of whatever origin), it is common to get an epenthetic oral stop b as a transition between the nasal voiced stop m and the following oral r (compare Greek *mrotos > *mbrotos (as in ambrotos) > brotos 'mortal'). 

There is of course also the curious Brahui denasalization of word-initial nasals but, as you note, this seems to be limited to before front vowel. So it doesn't look helpful.

I leave aside the question whether the attempt to link burušo with mūja/muža is felicitous or not. I am personally not overly enthusiastic about root etymologies like these. 

All the best,

HHH

The list may recall our discussion on Narmada and Narbada alternation, and the possible sound change where intervocalic labial nasal transforms to become an oral stop along the following lines: -m- > -β̃- > -v/b-. 

However, I was surprised to find a word-initial alternation of m and b in one of the famous papers Witzel wrote in 1999 (he was very prolific in that year). He  says the following in this paper (http://www.people.fas.harvard.edu/~witzel/MT-Substrates.pdf): 

North of this area, at the northern bend of the Indus (Baltistan/Hunza),  Burushaski is spoken. However, the language and the tribal name are indirectly attested in this general area ever since the RV: *m/bruža  (mod. burušo) > Ved. Mūja-vant, Avestan Muža.

Is such m-/b- alternation a known Burushaski phenomenon? Witzel seems to suggest that Mruža was the the ancient self designation of the Burusho- people but, unfortunately, without any helpful references.

I appreciate any information or pointers on such transformation.

Thanks,

Suresh.

It is interesting that Brahui, a Dravidian language now found in the southwestern corner of Pakistan, also shows a similar transformation:

*mē(l)-  'over, above' > Br. bē-

*miṭ- 'to leap' > Br. *biṭ- 'to throw, let drop'

However, this change is apparently conditioned by a following front vowel (Dravidian nasals in Brahui, Krishnamurti 2001:121-126), and perhaps unrelated to the above alternation.