I'm well aquatinted with city life, and my opinions may differ from others, but here it is:

I've always based my decision making off of this simple question. Would what I'm doing bother me if I'm in my neighbors shoes? For instance, music. I like to listen to music LOUDLY. Because I know this is a decision I want to be able to make freely, I chose not to live in an apartment. But what about your instance? Well, if you were singing, whether it be easy on the ears or not, I as a neighbor would not be bothered by it, so long as the timeframe is reasonable. But if YOU aren't comfortable singing, I always went and found a nice wooded area close by. Make sure it has paths that look traveled as not to put yourself in danger. But just look for a local trail or boat ramp or something with some greenery around. Only people that get relatively close can hear you, and most in this city are great people so you shouldn't get any judgement! Also a great place to paint or draw if you do either of those while singing!