I’ve heard Yuan-ti pronounced in various different ways by different people or video games (but some of these would have been English people and some would have been American, so there’s that aspect to add some confusion to this as well).
For the first part–”Yuan”–I’d pronounce that something like “you-arn” (I’m English, so I don’t pronounce “r” like Americans might). The obvious alternative is “you-an” (as in the name “Anne”).
It’s the last part–”ti”–that confuses me the most; is that “tee” or “tai”?
Is there an official canon pronunciation of Yuan-ti out there somewhere?