Sanskrit v- > S.E. Asian b-

I have raised this at the Austronesian Yahoo group, but I have only
had an incomplete response there.

In S.E. Asian languages such as Thai and Malay, many words
ultimately deriving from Sanskrit words in v- show b- (in Thai, this
has subsequently become ph-). Other Sanskrit words in v- yield w-.
Thai in particular shows a great many doublets. The explanation
usually given for Thai is that the words in ph- derive from Sanskrit
through Pali.

However, I have recently been checking on the etymology of these
words, and I have found that the Pali forms of these words actually
begin v-. Where then does the initial ph- or b- come from? I
suspect the answer may be quite trivial, but I cannot lay my hands
on it.

The change v > b does occur in Pali, e.g. deba < deva 'divine
(being)', nibba:na < nirva:n.a 'extinction', but is not as thorough-
going as the S.E. Asian forms would suggest. (I use capitals for
retroflex consonants, and quote Thai spellings as though the words
were Sanskrit. G = velar nasal in transliterations. A few minor
details of Thai spelling get lost. Thai tones are omitted, as these
loan words don't need tone marks.)

Examples are:

Skt.: _viha:ra_ 'temple'
Pali: _viha:ra_ 'temple'
Malay: _biara_ 'monastery'
Thai: _phiha:n_ "biha:ra" 'temple'
Thai: _wiha:n_ "wiha:ra" 'shrine, abbey'

Skt.: _vis'a_ 'poison'
Pali: _visa_ 'poison'
Malay: _bisa_ 'poison'
Thai: _phit_ "bis'a" 'poison'

Skt.: _vac_ 'tell, speak'
Malay: _baca_ 'read'

Skt.: _vacana_ 'speech' (and many other, related meanings)
Pali: _vacana_
Thai: _phot_ "bacana" 'word, speech'
Thai: _wacana_ "vacana" 'saying'

Skt.: _vaMs'a_ 'bamboo, race, shaft, flute'
Pali: _vaMsa_
Indonesian: _bangsa_ 'nation' (Apparently the Western Malayo-
Polynesian form)
Malay: _wangsa_ 'caste'
Thai: _phong_ "baGs'a" 'family, lineage'
Thai: _wong_ "waGs'a" 'family, lineage'

Indonesian forms in w- include Wisnu 'Vishnu', Waruna 'Varuna' and
_wira_ from Sanskrit vi:ra. The Thai forms of these words also
start w- rather than ph-.

Richard.