I bought my Spy Hunter upright at an auction.  The music control was turned down on the machine, and when I got home and turned it up I quickly found out why.  The music was distorted, scratchy, and garbled. Here's a video that does a better job explaining.

The problem turned out to be the digital to analog converter at U10 (AD7533JN). A socketed chip! They have apparently been discontinued, but I was able to find one at an electronics surplus place.  The chip is located at the arrow in the photo below.  A quick swap, and everything was back to normal!