I had the exact same problem where the game over sound (which slowly fades out) would not play. I fooled with it for weeks with no fix. Checked connectors. Lines 1-8, etc. In desperation, I subbed in the identical sound board from spirit (which does not have speech) and guess what. Mgow now plays the game over sound and spirit has no missing sounds!! I'm sure someone smarter than me can figure out from all that what's wrong with the original sound board but I cant...Hope this somehow helps!
By the way, all boards in mars were original gottlieb.

It never had the ending sound since I bought it. Never figured out what the problem was, but I rebuilt all the connectors so the problem was definitely on the sound board.