The audio board plugs into the game board, which is mounted on the cocktail door. As you close the door the audio board gets closer and closer to the monitor and once closed it is real close. I tried various suggestions from KLOV, like a faraday cage, with no success. In the end it cost $7 for this ground loop isolator, which has removed 95% of the buzz.
The ground wire of the plug connects and jumps around to every piece of metal in the cocktail. However, I never was able to pinpoint a jumped wire to the monitor chasis. There are quite a few areas that I am unable to see, so it's possible it's there and just not visible.