A couple of suggestions - check the Mute switch. Although it's only supposed to mute system sounds - such as new email etc - several Members have reported that, in some apps, it appears to mute the sound of that app too.
If that doesn't work, I'd suggest completely powering the iPad down by pressing and holding the Power switch until the slider control appears and then accepting the offer to power down. When the screen's completely blank, press and hold the Power switch again until the white Apple logo appears and the iPad will power up.
See if this fixes the problem, if not, don't hesitate to get back to us.
Tim