High Speed uses a "Background Sound Board" for the background sound. The in game sounds are generated on the CPU's sound section.

So, the first thing I'd be doing is re-seating all the cables to the background sound board, and the other side of them on the CPU.

Next, Make sure J1 on the Background Sound board has the following on the pins:

Pin 1: +5 Volts
Pin 2: Ground
Pin 3: -12 Volts
Pin 4: No connection

If all this checks out fine and you've re-seated the plugs, then it may be the MC1458 Op-Amp (U9), or the MC1408 DAC (U1). At this point you will need to test it in another machine, or on a test bench.

It may also be the IC sockets. Williams were still using the cheap sockets at this stage, so it would be best to replace them. It's the first thing I do after the initial testing on my test bench.