On the Williams system 11 games, the factory installed a 47 ohm @ half watt resistor in series with the volume control's shield wire. This prevents the volume level from going below a certain point (usually it's still too loud). The cure is to remove and bypass that resistor which is located right next the volume control next to the tilt bob. The shield wire will connect right to the bottom terminal of the pot where the resistor was located.
This will restore the full range of volume control so you can even turn it all the way off if desired.