Where can I find a reminder of the rules for pronouncing "oi" in French lyric diction? I need to sing the word "voix", so I looked it up on Wiktionary, which unhelpfully gives both pronunciations ("/vwɑ/, /vwa/") without any explanation of when, where or how each pronunciation is used. Which would be correct for use in an art song?
In France the usual pronunciation of oi is /a/ as in [ʁwa], [vwa], [mwa]. The difference between /ɑ/ and /a/ is highly lost in contemporary French. What you can hear on Wiktionary is [vwa]. On the French side they only give the [vwa] IPA transcription.
Hearing [vwɑ] in an opera would probably not disturb me, I might not even notice it. I don't know much about lyric diction but I presume you need to obtain the correct number of syllables, so pronouncing oi as two distinct syllables such as [vu.a] or [ʁu.a] would not surprise me either.
As a side note I want to point out that in Québec they still pronounce the letters oi /wɛ/ as we used to do in France a few centuries ago.