Some apps misinterpret the iPad's Mute function (which is only supposed to mute system sounds) and will cut off all audio if that function is enabled. There are two potential ways in which it can be switched on - either the hard switch next to the volume control or the soft switch in the far left of the multi-task bar. It might be worth checking. Mute does not affect headphone so a quick way to see if this is the problem is to plug in a headphone and see if the volume returns. If it does, it's almost certain that the Mute function is the issue.
Tim