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Sounds like Mute. This can be turned on/off either using the slide switch next to to the volume buttons, or in the multitask bar. See below link for info on how to check you settings for the side switch.
iPad: Understanding the side switch
Contrary to common sense, Mute does not stop all iPad sounds. It only stops system sounds like; key clicks, alerts, ringing, etc. Music, video, and most game audio will are not affected. It's mostly about stopping the iPad from making awkward noises at meetings, the theater, or during grandpa's fishing stories.
That said, my system sounds did stop the other day, and Mute was not on. A restart fixed it right up.
From general settings change switch properties from orientation to mute.
Then manually mute the ipad from the physical switch.
Then unmute from physical switch.
In game sound should now be working.
It's a bug...