I have a spare Pinnovators Pin-Pac 1 headphone jack that I will no longer be using. For those not familiar, this jack installs allowing you to plug a set of headphones into the game and operate without external sound. You can read about the jack here:
http://www.pinnovators.com/index.php/featured-products/pinpac-1-system-description
This is easy install (5 minutes) and requires no permanent changes to the machine. Your machine does need to have the door/filler for a dollar bill acceptor (not the bill acceptor itself - just the panel) Consists of three parts. An adapter that attaches to the sound board, a cable and a board that replaces the panel in the door. The board has a toggle to allow you to switch between the jack and external sound as well as a volume control and the jack. Lightly used and in perfect working order. Check the website for exact compatibility with your game, but a partial list includes: RS, STTNG, CFTBL, FT, TZ, BSD, TAF, DM, TOM, WCS94 yada, yada (check the website for your machine)
$75 shipped in CONUS. PM if interested.