There are only two things that control the the keyboard click. One is Mute (not always the side switch, and covered in my previous link), the other is the the settings of Settings > General > Sounds. Here you must have it turned on. Also notice that you can make the system sounds volume independent of the main volume level (Change with Buttons: OFF), or have it tied to the iPad's overall volume level (ON) In the second case if you've turned the iPad's volume down too far you won't hear the system sounds.