Sherlock never sings in the show, and I doubt the character was meant to be good at singing. But I always imagined he would have a pretty good voice. I also imagine that he would do something like singing a song to someone in a language he is fluent in, that he knows they don't understand, to convey a sentiment or message, just to get it off his chest, or just to amuse himself by confusing and vexing them and making them wonder what he is saying.