Have you got notifications muted ? Only notifications (including games) get muted, so the Music and Videos apps, and headphones, still get sound.
Depending on what you've got Settings > General > Use Side Switch To set to (mute or rotation lock), then you can mute notifications by the switch on the right hand side of the iPad above the volume switch, or via the taskbar : double-click the home button; slide from the left; and it's the icon far left; press home again to exit the taskbar. The function that isn't on the side switch is set via the taskbar instead : http://support.apple.com/kb/HT4085
Try this - Reset the iPad by holding down on the Sleep and Home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons. (This is equivalent to rebooting your computer.)
Check your settings. The iPads have a small switch on the right edge. It can be used as a rotation lock to keep the screen from automatically reorienting itself as you move around, but you need to have the tablet’s settings configured properly. That same switch, right above the volume buttons, can also be set to function instead as a mute button to silence certain types of audio.
If the switch is set to work as a mute button, you can change its purpose to “screen-rotation lock” by tapping the Settings icon on the home screen. On the Settings screen, tap General on the left side, and on the right side of the screen flick down to “Use Side Switch to.” Tap to select Lock Rotation or Mute to set the button’s function. Even if you set the side switch for your preferred use, you can still mute the Mini or lock the screen. Just double-click the Home button, and when the panel of apps appears along the bottom edge of the screen, flick the row from left to right with your finger. Tap the icon on the far left side of the row to either lock the iPad’s screen or mute the iPad’s alerts, notifications and sound effects. Music, podcasts and video are not muted unless you turn the volume all the way down.
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