Make sure you have not muted the iPad. This will be either the side switch or an icon in the Control Panel; depending on your settings in Settings > General > Use Side Switch to: Lock Rotation / Mute.
That's for iOS 7. If you are using earlier versions of iOS the controls and settings will be a bit different, but do the same thing.
Muting the iPad is not the same as turning off all sound. It is supposed to prevent alarms, alerts, ringtones, and other systems sounds that could potentially interrupt a meeting or be socially awkward in certain environments. It will not normally stop sound from playing in music, games, or most other apps; though this is up to the developer.