The little side switch on the upper right hand side of the iPad can be adapted for two uses: mute and lock screen.
By default the side switch is set to mute. Some prefer lock screen. To change: Settings > General and you will find the side switch options.
A user may change their side switch to lock screen, forgetting then that they may have the mute turned on in the multitasking bar. Music works fine, videos play with sound but the games have no sound.
It's important to remember: the mute only mutes: notifications, game sounds, alerts, sound effects..... not video, music, movies.
No rotation? Same situation - user has the side switch for mute, forgetting that they locked the screen in the multitasking bar.
Apple support on this topic: