Well first of all, check to make sure the sounds are activated in settings.
Next....they haven't inadvertently flicked the mute switch on? This is found just above the sound toggles on the right top of the iPad.
That mute switch can have 2 different functions and if it has been changed recently and done in a particular way, this can then mute your iPad, so check in Settings - Use Side Switch To......there are 2 options, lock rotation and Mute. Has this been changed recently?
Have a look at the sound slide which is found by doing the 4 finger up swipe or double press the home button to reveal the Multi task bar at the bottom, swipe to the right and you will find a sound slider here.
Also have they been using headphones recently? Some headphones have a mute switch on them. Also if headphones have been used, anything you have been listening to, needs to be first stopped before pulling out the headphone jack, otherwise this will mute your iPad. Another thing to look for is if any debris has been caught in the jack at all. Have a look and a vacuum can be applied if you can see any debris in there.
I hope this helps...let us know.
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