This happens to me sometimes when I plug my iPhone into my car.
Try this to see if it fixes it:
1. Stop the music.
2. Press the Volume Up and Down buttons. Does it show "Ringer"? If not, it's stuck in the "other mode". (This verification may not work if you have 'Change with Buttons' turned off in General > Sounds).
3. Go to Ringtones.
4. Change your Ringtone and then change it back.
5. Play music. Is it fixed?
This used to happen to me rarely on iOS 4, and strangely it would only happen sometimes when my iPhone was plugged into the car and a few times when playing Tiny Wings. The phone would get stuck in another mode of audio output that sounds like crap.
Try this to see if it fixes it:
1. Stop the music.
2. Press the Volume Up and Down buttons. Does it show "Ringer"? If not, it's stuck in the "other mode". (This verification may not work if you have 'Change with Buttons' turned off in General > Sounds).
3. Go to Ringtones.
4. Change your Ringtone and then change it back.
5. Play music. Is it fixed?
This used to happen to me rarely on iOS 4, and strangely it would only happen sometimes when my iPhone was plugged into the car and a few times when playing Tiny Wings. The phone would get stuck in another mode of audio output that sounds like crap.