Give each person a folder in which to stick whatever loose sheets they are currently working with. Keep these folders plus whatever music books are relevant on a shelf. Tell each person to put their shit away when they are done.

There can be another folder for sheets people fail to put away. Your wife can just stick everything left out into this folder when stuff gets left out and she gets annoyed. People will learn they have to look through all the shit in this folder to find their stuff it they leave their stuff out.