I'll spontaneously burst out into song on occasion, but for the most part I don't do it. In my maths class a few years ago my friends and I always used to start singing - albeit quietly - whenever we finished our work, or sometimes even before then. Now I just quietly drum my fingers if I have a rhythm stuck in my head, because it's generally less annoying for the people around me (although nobody has ever told me to shut up whilst I'm singing).

I generally have no complaints when a person sings though, unless they're tone deaf and/or they only sing the same line of a song (my mum is guilty of both, and it drives me insane).