Thanks for the suggestion gaz - no joy for me, it as just a louder crackle. I have since found the following - it's not been any use for me (still not working) but might work for somebody else who has the same problem.
Problem: Sound was static noise, and then nothing.
Answer: Changed prism cards from my other one and it works fine.
Check to see if the game passes the POST for the sound section (it'll flag it in the error report if not). If it does pass, the DSP chip is probably Ok and look onwards from there for other faults. It's likely a very simple fix - either a simple little opamp or a bad connection. If no error report, then the DSP chip is likely okay, so suspect something in the output. Unfortunately, there also isn't anything *between* the DSP and the output connector, so it may be the DSP chip after all. (Jonathan Deitch)
When swapping RFM to SWE1 the sound might no longer work and you might see the message U109 error! This is no hardware defect but a Star Wars problem. You need the latest SWE1 sound software to be loaded and the the sound works fine again.
Problem: Static comes on all three speakers during attract mode, particularly when there was no background sound and white text was displayed.
Answer: There is "crosstalk" on the cable group between the PRISM board and the amplifier board. This cable group is several cables, which are tied together with nylon tie wraps. Cut and discard the nylon tie wraps (separating the cables), and the static problem should go away.
If the static is still present, changed the 04-12621 audio amplifier board. The audio amp is a small board inside the backbox's computer box, which connects to the PRISM card. Sometimes the old amp, when failing, can cranking out way too much volume even at the lowest settings.