First check the 'Slide Switch' (right upper side of the iPad) - this is defaulted to control the sound mute (or not) - make sure that the switch has not been unintentionally set to mute; second, if the first suggestion is not helpful, then open your multitask bar (i.e. double tap the home button or use multi-fingering gesturing); swipe right until the brightness & sound controls appear; make sure the sound is not muted.
If the above is not helpful, the 'Restart' and if not effective, 'Reset' the iPad - directions HERE, if not familiar w/ the maneuvers - good luck and please post back if none of these suggestions help!