If you've tried the other suggestions in this thread you may have the system alerts muted. Go to Settings-General and set the function of the slide switch to Mute. Then move the slide switch to unmute your IPad. You can then set the function of the switch back to lock rotation if you want.
What can happen is that if you set the function of the slide switch to mute, then slide the switch to the mute position, it does't reset back to unmute just by changing the function of the switch back to the lock rotation - the system stays muted. And, there is no icon to tell you that you have the alert tones mutes. Music still plays, but no alert tones.