You may have system sounds muted. But first make sure that you have sounds enabled in Settings>General>Sounds.
You should also check the side switch is not in the "Mute" position.
this is my husband's iPad - so here goes.. sounds is not under general - it is just under sounds. side switch is not on mute. in ringer and alerts the volume is up. sent mail has selected "swoosh". I changed the sound to sherwood forest and sent an email from his iPad and still no sound for sent mail.
so does this end my session?