i dont think it works with all apps, but for a lot of them you can do it using this method, if the apps have a 'settings' option, you can usually turn the app sounds/music off, then double click the home button in order to access the music functions.
applegenius,
if an app has background music normally, then replacing it with something of your own is sometimes preferable. and some of us can multi-task, listening to music AND using apps! 😝 lol.