You’re listening to music on your iPad and decide you want to play some games. But when you start Temple Run there is no sound. Go to another game – same thing. Back to the music and you have sound again. What gives?
At some point you double-clicked the home button and hit mute. Or in Settings you changed the setting of the external slide switch from mute to orientation lock. Simply, go to that setting and change it back to mute and then back orientation lock. Ta Da! The sounds of race car engines, jungles, and birds whistling toward their target have been restored to their former glory.
Play on.
Source: apple.com