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



On 17 Nov 2012, at 10:22, Suresh Kolichala wrote:

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.