It is highly likely that your sound board needs professional repair.

It's quite possible that the yellow tantalum capacitors located next to the audio amplifier outputs are incinerated.

Sound board repair can be tricky.

Coin Op Cauldron has a flat rate $125 repair on these, and though it might take a while to get back, I'd consider that a bargain.

Capacitors have a lifespan. I always figure about ten years. Considering that your board is thirty years old... there may be a whole host of capacitors that need replaced.

If you are doing your own work, consult the pinwiki:

https://pinwiki.com/wiki/index.php/Williams_WPC#DCS_Sound_Board_Issues

Consider replacing all the electrolytic capacitors. Definitely consider testing/replacing all the yellow tantalum capacitors on all these Williams sound boards. There are some chips that are frequent failure points. My experience is that problems on these sound boards aren't as easy as 'if I put in a cap kit it'll work', but your experience may be different.

I do a lot of my own board repair, but this is one that I'd gladly send off.

OP: If you choose to do this with the radial snap-in capacitor, please take care not to pull the through hole. If you do pull the through hole you will need to restore it (stitch) or install a jumper to the appropriate locations.

Agree. I hate this board. Particularly the OEM board. The traces and pads on the OEM board are tiny and weak. Very easy to remove pads and pull traces.

Have you conducted the test that I described for you on another forum?
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I've lately (past three years) seen a lot of tantalum capacitor failure in the Williams Pre-DCS sound board. Good pictures in this thread:

https://pinside.com/pinball/forum/topic/trying-to-fix-wpc-a-12738-sound-board-t2

I love the second picture in this thread. It shows clearly a view from the top of the heat sink. A tantalum capacitor is the yellow looks-like-a-resistor, and just above it is a failed tantalum capacitor that has incinerated itself. Further pictures in this thread show replacement tantalum capacitors installed in this Pre-DCS board. Hint: Look for yellow.

Tantalum capacitors have better temperature stability than inexpensive Electrolytic capacitors, but they fail differently. Electrolytic capacitors fail (usually) by the electrolyte drying out, or chemical failure that forms gas that vents out of the top. They will fail usually by just not reading the full value of their component (a 1000uf capacitor reading 816uf or even open on a good capacitor meter), or they will increase in ESR which requires a good capacitor meter:

https://www.amazon.com/Peak-Electronic-Design-Ltd-166225/dp/B005NIBEYU/ref=asc_df_B005NIBEYU/

When they fail by increasing ESR, sometimes you can feel the heat difference with your finger. A hot electrolytic capacitor is a likely candidate for replacement.

Tantalum capacitors can also change in value, but they usually fail by creating a dead short! Which creates a fire that incinerates the part. I've seen both failures in Pre-DCS sound board capacitors, the change in value and the charred remains.

Tantalum capacitors don't have to be yellow painted, but the supplier Williams used makes them fairly easy to spot. Tantalum is frequently used for values of 1-22uf. The tiny yellow bead capacitors that are .1uf, .01uf. .001uf, etc. are probably ceramic, so not every yellow capacitor on these boards is tantalum.

The parts list for your board would probably specify which are tantalum capacitors, but I did a quick look online and didn't find it.

Here is a link to something interesting:

https://www.bestofpinball.com/catalog/index.php?main_page=product_info&cPath=22_271&products_id=5753&language=en