An answer to this can't be straight and simple. If you have purchased the songs online or from a cd store, you are entitled to have one legal "backup", provided you aren't sharing, or selling the "backup". If you lose the cd, you can continue to use the "backup" until you re-purchase the lost item. If you DL a file, mp3 or otherwise without having purchased it, then yes it is illegal. I'm no law student, so correct me if i'm wrong.