It probably depends on the city, but in general the legal noise limits allowing for "quiet enjoyment" seem to be pretty high. I lived above loud college student

It probably depends on the city, but in general the legal noise limits allowing for "quiet enjoyment" seem to be pretty high. I lived above loud college students for awhile in SF, and was curious whether they were technically breaking the limits, so I downloaded a decibel meter app to check. Even with the phone sitting on the floor, they would only break the limits when someone yelled at the top of their lungs or they played extremely loud music. That was despite being able to hear pretty much all their (loud but not shouting) conversations, (loud but not usually extreme) music, etc. very clearly in our apartment. So I think singing should be fine legally speaking unless you live somewhere with a lot stricter limits, especially during the day. It's a good idea to check with neighbors though. Often they won't mind if you do it as long as you avoid certain times or certain rooms. That's how we (sort of) resolved things with our student friends... and thankfully we moved out of there pre-quarantine :)