Womble

Hi Pete
Probably the simplest thing to try is to remove and reseat the 3way connector board that links all the PCBs together.  That can chase away intermittent issues. Eyeball the contacts when you put it back on to ensure the contacts are properly lined up. The smaller of the three boards is the sound board.
If you still have issues after that the next stage is to use the audio test mode. Star Wars uses four POKEY chips for the audio, two for the music, two for the sound effects , so losing one can take out certain SFX or leave the music sounding a bit odd if one of the music ones has gone.
You'll need to pull the PCB out so you have easy access to the sound board and a short alligator clip test lead.  On the inner edge of the sound board you need to find 2 test points, labelled "TP11 GND" and "TP7 SELF TEST". Connect one end of the alligator test lead to the TP11 GND test point on the sound board, make sure the other end isn't touching anything yet.
1) Flip the test mode switch above the coin box
2) Power up the machine
3) Using the Yoke controller to step through to to the Switch Test  on the screen
4) Connect the other end of the alligator test lead to the SELF TEST tag on the sound board
The board will then start to play through the audio test, and will test each POKEY by playing a pattern of tones
POKEY #1 - Single Beep x4
POKEY #2 - Double Beep x4
Slight Pause
POKEY #3 - Triple Beep x4
POKEY #4 -Quad Beep x4
If you are missing one of them then you either have a bad POKEY IC, or it is in a bad socket.
It will then go onto the speech clips, and will eventually step through the music clips. If you heave the lead connected it will only play a short bit of each tune before moving on, but you can disconnect the lead and every time you tap the test point it will progress to the next item in the test list.
That should tell you if everything is working fine or not. Bad sockets or logic faults are also fairly common, as are dead 6502 CPUs or PIAs, but the fact it works partially means there isn't anything major wrong. Hopefully its not a bad POKEY, those are getting expensive.