I have written some test ROMs from scratch that can be run in the game. You may find the 13-sounds ROM useful. Give it a try and see if it helps you find the problem. You could use it to trigger a repeating sound, and see if it plays for more than ten seconds without touching the machine (the repeating sounds are triggered only once from the ROM, and then nothing else goes on inside the game. The sound card should play forever or until you send a Cancel).

You can download them all in a single zip file from here: http://pincoder.reversion.ca

There is also a thread on this site where you can leave some feedback: https://pinside.com/pinball/forum/topic/new-williams-system-6-in-game-test-roms

Let me know how you make out!