I used to work at a place where there were a lot of transgender prostitutes outside in the evenings. They would proposition me as I walked by (as well as flash their parts at me sometimes). Some were pretty convincing regarding looks but when they spoke you could tell.

(I said "limited experience", but I suppose I probably have a fair bit more experience than most people).

It doesn't matter if a few are good at convincing, as long as most are unconvincing. Unless the people who are good at voice conversion are the same people who have done body shape modification aand wardrobe modification and are opposite the people who openly admit about their transness, it's easy to estimate what fraction of trans people are good at changing their voice.

a semi-related question, can people with training in music recognize the difference between a sopranist and a soprano?

because as a layperson i really can't.