Do you hear keyboard clicks and other app sounds? If you don't you have system sounds muted. Before you check that, make sure that you have sounds enabled for games and Facebook in Settings>Notifications. If that's not the problem, then system sounds probably are muted.
Settings>General>Use Side Switch to: Mute System sounds. If this option is selected, the switch on the side of the iPad above the volume rocker will mute system sounds.
If you choose Lock Screen Rotation, then the switch locks the screen. If the screen is locked, you will see a lock icon in the upper right corner next to the battery indicator gauge.
If you have the side switch set to lock screen rotation then the system sound control is in the control center if you are running iOS 7 or iOS 8, Swipe up from the bottom of the screen to get to control center. Tap on the bell icon and system sounds will return.
If you have the side switch set to mute system sounds, then the screen lock can be accessed in the same manner as described above.
This support article from Apple explains how the side switch works.
http://support.apple.com/kb/HT4085