music is definitely noise

If you guys have never played musical instruments durinng the designated "quiet hours", then the key focus is in the first sentense. If there are older people or young kids living next to you, playing musical instrument (any time) could be come a nuisance that means nothing to you but incredibly annoying for your neighbors. And yes, music (even if you play Mozart CDs) is definitely noise because it disturbs others and could easily interrupt their daily routine.