Social Norms: Making Noise

I appeal to the hivemind, for somebody more perceptive in this area. What are the 'rules' about singing in public? It seems to be far more restrictive than 'when is it acceptable to have a PA system or a radio play music at people in public' even at the same volume levels. But then again, I'm afraid of my own voice and so my instinct is that it is nearly never acceptable to sing in public.