Guest is not treated as a normal account, so it would be for a user you don't want permanently in your system. Someone who browses, checks their email and whatnot, and once they go back home you don't want to go thru the hassle of deleting that account and creating it again. Files created while using a guest account are deleted when the user logs out. A temporary home folder is created for the guest’s files. When the they log out, the home folder and it’s contents are deleted. A guest user cannot change other accounts or alter the settings on the computer.
You can either 1) create an account which is not temporary for your guest user, and make them store their music there (and delete that account when needed), or 2) use some alternative solution to having music, such as Pandora.