I've been dealing with this same problem on my Black Knight intermittently since I bought the game over 15 years ago. The sound would intermittently change to wrong sounds during gameplay (the most noticeble was the "all 3 drop targets collected" sound and the bonus count sound were both dramatically different than they should have been), the background sound would stop running, and during attract mode it would also intermittently start making the same ever-increasing sound effect (pitch would increase with each cycling of the displays between last game score and high score to date) just like 
wrd1972_PinDoc showed in their video.
Even the speech would still work but sometimes say the wrong phrase. My favorite example: just before I finally decided to give this problem another stab, the game-over speech had changed from "One Enemy Cannot Fight the Black Knight Again" (or whatever it's usual phrase) to the Knight's laugh!!! It was the funniest error I've witnessed on a game! It was also the final slap in the face that motivated me to finally slay the problem once and for all.
I first tried reflowing solder to the MPU connector 1J8 and re-pinned the female connector. That seemed to help but the problem still returned eventually (though less frequently). I tried replacing IC36 yesterday and I thought it was finally fixed... until this morning when it started back to it's old tricks and laughed at me again at the end of the game. This time I pulled the boards to double check my work on IC36 but also went ahead and *replaced* the header pins at 1J8...
I think it's *finally* working properly now, after 15+ years in my possession. I had it on for hours today without a hitch. Thank you 
PINTEC for the sage advice and wrd1972_PinDoc for posting this thread. Hopefully yours will also be fixed soon!
Richmond, KY
