Things That Go Ding in the Night
You’ll get a notification chime when mail arrives. When your iPad is on a network (Wi-Fi or cellular), this chime can sound whenever new mail arrives, even in the middle of the night. It’s not loud or annoying, but it can be baffling if you don’t know where the sound is coming from. To turn off this mail delivery sound go to Settings > General > Sounds. You can also control other items such as keyboard clicking when you type and the lock and unlock sounds of your iPad.
Excerpt from my eBook iPad Academy: Tips, Tricks and Clever Techniques