Because of a bug in iOS 7, you might have to delete songs from your iPhone, and probably all of them, through the settings. No worries though, because all of your music can be backed up through the iCloud that saves the information in Apple’s servers. First go to the settings app, click on General then Usage. After the Storage section at the top loads, find Music in the list and tap on it. Choose Edit and then the Delete option to delete all music from your phone. From here you can re-download your music from the iCloud, but they should fix the bug in the phone’s system.