well, if you're flats are situated such that one's loud music can disturb the other, i think you should think about having some sense of others.

if you want to play or make loud music, keep in mind also that perhaps the neighbors would like to rest on weekends and have some peace and quiet.

my flat allows me to have very loud music, and the neighbors don't hear a thing. so i am not a nuisance to my neighbors. but if they could hear the loud sound, i would be more considerate, and turn the volume lower.