As a theatregoer, your only job at the theatre is to sit in your seat and watch the performers on stage. That's it.
Anything else is disrespectful the cast, orchestra, crew, FoH staff and your fellow audience members, it is completely arrogant. If you want a private performance where you can sing along to your heart's content then book and pay for one, don't spoil it for everyone else.
Singalongs/reprise/RH, etc, aside.