When it comes to setting music as a ringtone, Android generally sucks. There is no easy or consistent way to set music as a ringtone across devices. You often have to open the stock music app and figure out how to make music a ringtone, then set the ringtone using a separate process.
The Rings Extended app, created by Diane Hackborn, adds the ringtone functionality that should be on all Android devices. It acts as a ‘middle man’ ringtone manager whenever you try to set a ringtone from areas like Settings > Sounds or Contacts > Edit > Ringtone. No need to explicitly make that favorite music track into a ringtone first. Rings Extended will make your entire music library or any other sound on your device an available ringtone.