you absolutely can sing loudly in a theater, but you don't have a right to demand you can keep on doing it. It's a private location, so they have every right to throw you out and ban you. similarly to how you don't have a right to say whatever you want without being moderated on Twitter or Reddit.

the fire one is basically anything that incites panic can get you into legal hot water, and if there are injuries or death as a result some form of manslaughter charges probably because ultimately you were responsible.