You can turn the game and interface sounds on / off separately.
- If you want to turn off game sounds, just tap on any game you want and start it. On the top left side, you will find a "II" symbol, which will stop the game. Once you tap on it, a menu will appear (Restart, Help etc.). At the very bottom you'll find the option to turn the sound on and off.
- If you want to turn off interface sounds as well, just go to the 'Me' tab (then tap on View and edit profile for iOS) and scroll down to find the option to turn the interface sounds on and off.