Some apps don't have sound
Some apps doesn't play sound, there is sound when i plug in earphone
iPad (3rd gen) Wi-Fi, iOS 5.1.1
You likely have system sounds muted.
1. The switch just above the volume control on the upper right hand side of your iPad can be configured to lock orientation or mute in settings. So, the first thing to try is to move that button and look at your screen and it will either lock orientation, or mute. If it says mute, you're done.
2. You can change the setting in Settings OR
3. Double tap your home button to bring up the task bar (most recently used apps. Swipe them all the way to the right revealing the last icon on the left, which will be a mute button. Touch it and you'll restore system sounds.
Tap your home button once more to close task bar. Enjoy sounds on your iPad