Is it normal for people to sing in an apartment building?

Singing is fine just don't keep your neighbors up

singing is good.

If no one's complained, then they either can't hear you or don't care.

you play any instruments?

Can you hear other people through your walls? If not they probably can't hear you.

If they don't complain don't worry about it. Just don't sing ode to joy or whatever at 3am.