I have a strange one for you. Out of the box, the iPad mute/rotation lock was on mute and was muted. I went into settings and choose rotation lock. An hour later I was showing off a retina ready apps and there was no sound. I checked several other apps and no sound. I went to sound in settings and chose a different sound and it played. Then back into an app and nothing !?!?
Right before getting ready to restore, I went in and switched back to the mute option then switched the switch from mute on to mute off a few times. I then left the mute off and went back in and changed the setting back to rotation lock. Every since then, every app sound has played just fine.
So if anyone is having sound issues, you might want to give this a try.