Have you tried adjusting the sound pot on the board? Bottom right side of the board, next to the speech pot. Cleaning those pots often helps. Remove board, pour a small amount of alcohol on the pots, twist pots both directions several time, blow dry or air dry. Use 91% isopropyl alcohol, lower concentration takes long to dry/ evaporate.

https://www.pinwiki.com/wiki/index.php/Bally/Stern#Squawk_.26_Talk_Sound_Board_.28AS-2518-61.29

I might be wrong, but is the missing sounds MPU driven?

I had the same issue on my FB II a few years ago, I replaced the pots and the caps on the squawk and talk in the kit from Big Daddy and problem solved

Almost all "sounds" on that board come from the AD558 DAC.
The OEM sockets for the AD558 are always shot.
I'd start by replacing that socket.

Also replace the trim POTs. You might get some signs of life by simply twizzling the sound POT.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
https://www.youtube.com/c/ChrisHiblerPinball - My YT Channel
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The pop bumper and slingshot sounds are produced by the AY-3-8912 sound generator chip.
The background sounds and drop target sounds are produced by the AD588 DAC chip.

The outputs from both of these chips gets mixed together and are controlled by the sound volume trimmer pot on the board.
Since you're getting AY-3-8912 sounds, the sound volume trimmer pot is unlikely the issue.

My money is on the 2uF capacitor at C43 going almost open circuit which is a common failure on these boards. Infact you should replace all the capacitors on the board.

Great info Quench
Ya. If the board hasn’t been recapped, it should be. That is step one for me with all S&T boards.

Next…but not necessarily contributing to your issue…
…sockets
…POTs
…test 6821, 6808 in other boards.
…jumpers valid
…ROMs

Be very careful with the TMS5200. Don’t touch it if it is working. The legs in those are almost always brittle and can break off easily.
—
Chris Hibler - CARGPB #31
Http://chrishiblerpinball.com/contact
http://www.PinWiki.com/ - The new place for pinball repair info

Also worth mentioning that the DACs fail too. I have repaired quite a few that did not work after capping them and then discovered a faulty DAC. I keep a spare S&T handy with all socketed chips. It's great for troubleshooting these exact issues.

Since you can "barely hear" the sounds, the DAC is probably working... So the capacitor being faulty makes sense here.

Not sure if the info is useful for PinWiki but I can go through the S&T games using the AY-3-8912 to determine what sounds come from the DAC or the AY-3-8912.