You can mute the Arduboy by holding the B button on startup. The Blue LED should come on. While continuing to hold B, press the DOWN button. The LED should flash red. Now you can release the buttons.
If you want to unmute your Arduboy, hold the B button again on startup. The blue LED will come on. Now, hold UP instead of DOWN. The LED should flash green and change back to blue. Now release all the buttons.
Please correct me if I am wrong. I am very new to the Arduboy .