I heard on the Wikipedia article for Sindarin (which I admit is far from being the best source) that Common Brittonic, like Old Irish, had a nasalized v sound ṽ. Is this true? If so, are there any other language that use this sound? I'm sorry if this isn't a valid question.
This phoneme /ṽ/ appears to be common to both Common Brittonic and Old Irish, and shows the difficulties that the contemporary scribes for Old Irish had with notating nasalisation. As of January 2021, Wiktionary transcribes it for Common Brittonic as /β̃/ but Old Irish with /ṽ/. The difference between the two being merely a notation difference is mentioned in Stifter's Sengoidelc: Old Irish for Beginners:
/μ/ is pronounced like /β/, but with a nasal quality. In other publications you may find this transcribed as /ṽ/.
/β/ is a labial sound as in Latin Vergilius. In other publications you may find this transcribed as /v/.
However, Gillies (2009:435) sees this as stages in diachronic change:
[μ] > [ṽ] > [v], [β] > [v]
It was by no means a rare marginal phoneme: it was used in words as common as *nemos "sky" and *temeslos "darkness". Its reflex remains a important part of the morphophonemics of the modern Celtic languages, occurring whenever lenition (soft mutation / séimhiú in Irish / treiglad meddal in Welsh) operates on the phoneme /m/.
Owing to the conservative nature of Irish orthography, it corresponds quite well to the spelling
mh in Irish and Scottish Gaelic, e.g. teimheal. In Modern Welsh, it is mostly written with
f, but a lot of these have
mu in Old Welsh. Breton uses
v for the form after lenition, although it frequently uses the digraph
ñv, indicating nasalisation of the preceding vowel; older forms vary between
Nonetheless, the modern reflexes of this consonant across Celtic languages vary between /w/ and /v/ (palatalised or velarised); only in Breton is the nasalisation retained, and even then, I'll let you all be the judge of neñv and teñval, and to what extent the spelling reflects modern phonetics versus historical etymology.
There have been studies about the nature of nasal fricatives; one 2016 paper states how they are "unusual, maladaptive articulations". More relevant to this question, there has been work done understanding to what extent
mh is different to
ph in Modern Irish and Scottish Gaelic. This is currently under investigation: a 2015 paper claims incomplete merger in Scottish Gaelic, implying some marginal distinction, and also cites two languages which have been claimed to have [ṽ]: Umbundu and Coatzospan Mixtec; the authors conclude it is phonetically a form of vowel nasalisation that spreads, turning the fricative into more of an approximant. On the other hand, one 2020 study concludes that there is no distinction between
mh, and that there is insufficient evidence for any nasalisation.