A couple of things you can try....
'Settings', 'iPod', 'Sound Check' -> 'On'
Check the Mute switch is not activated or, if you've changed the mute 'switch' to the multi-task bar, check it's not activated.
The iPad has independent volume controls for the loudspeaker and headphones so, if you've been previously listening on headphones and are now on loudspeaker (or vice versa) you might need to re-adjust the volume control.
If these suggestions don't work, don't hesitate to get back to us for some more ideas....
Tim