I had the same problem - if turning the sounds on doesn't work, you may have to have the sound board rebuilt... after 30+ years, those caps have seen better days.

I just finished getting a Flash game working 100% which included rebuilding the sound card and part of the driver board... details here - I should have done more video on the sound board rebuild, but basically I replaced all the electrolytic caps on it: http://pinballhelp.com/fixing-slingshots-and-coils-that-dont-fire-on-williams-system-3/