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If the link doesn't work, at the top of this page, click on support, then in the middle of the page manuels and find the Ipad one.
If you checked the volume rocker on the right side of the iPad (while holding it in portait mode) - above the side switch and still have no sounds in games or apps - this may be why.
If you lose sounds for keyboard clicks, games or other apps, email notifications and other notifications, system sounds may have been muted.
System sounds can be muted and controlled two different ways. The screen lock rotation can be controlled in the same manner as well.
Settings>General>Use Side Switch to: Mute System sounds. If this option is selected, the switch on the side of the iPad above the volume rocker will mute system sounds.
If you choose Lock Screen Rotation, then the switch locks the screen. If the screen is locked, you will see a lock icon in the upper right corner next to the battery indicator gauge.
If you have the side switch set to lock screen rotation then the system sound control is in the task bar. Double tap the home button and in the task bar at the bottom, swipe all the way to the right. The speaker icon is all the way to the left. Tap on it and system sounds will return.
If you have the side switch set to mute system sounds, then the screen lock rotation can be accessed via the task bar in the same manner as described above.
This support article from Apple explains how the side switch works.
You can also check in Settings>General>Sounds. Make sure that you have the various sounds turned on in the settings.
For what its worth, I had a similar issues and called the UK helpline & technical support number. A guy called Glenn was right on the button with what he thought had gone wrong and he was right. From my perspective, this was an issue with an out of warranty iPad, however, due to Glenn's product knowledge, he was able ascertain what had gone wrong during the iOS 3.2.2 to iOS 5.1 upgrade. This saved me time, money and has restored my faith in Apple as a quality brand. I suggest you call the UK help line. I previously called the American line and, no offense, but they were unable to fix anything. As is often the case, it comes down to skilled individuals who actually care about the company they work for. Call Glenn and his colleagues; they will out you right.