The book "The Vampire Lestat" makes it abundantly clear that Lestat could neither play the violin, nor sing with any great ability before he became a vampire;
But I was in paradise again. And so was Nicolas though no decent
orchestra in the city would hire him, and he was now playing solos
with the little bunch of musicians in the theater where I worked, and
when we were really pinched he did play right on the boulevard, with
me beside him, holding out the hat. We were shameless!
.................. [later]
Something to do with the sentiments Nicki was expressing. Didn't make
sense. I tried to shrug it off.
"If you'd set out to play the violin, you'd probably be playing for
the Court by now," he said.
Lestat is, however an accomplished actor with excellent vocal projection, a regular churchgoer and the second son of an aristocrat so it's likely that he's had a level of choral training and almost certain that he can play some kind of instrument (he repeatedly mentions the harpsichord).
For the record, the vampire Armand already knew how to play the violin before being turned into a vampire (or at the very least owned a violin) since it was listed among the belongings he takes with him.