My SRC resistor network parts came in so I pulled the MPU and rechecked each of the SRCs. A few of the pins had high ohm readings so I pulled U41 and U42 and checked them again. All of the ohm readings on every SRC checked out. I continued to verify the sound circuitry to the prints with my meter and everything looks great. I put everything back together and the booted up with a U42 error and a memprot. error. It rebooted a few times then it booted without error. There was still a missing sound at #15.
I pulled the soundboard and checked that as well, but I remembered that I replaced the original board when I first bought the machine because the sound wasn't working. The original board looked hacked with some resistors on the back. I dug it out of a box and figured It couldn't hurt to try installing it now that I pretty much ruled out the MPU and...
It worked. All of the #$%#&%* sounds work!!!
Now I know it's on the background soundboard somewhere and I can stop pulling out the MPU. I completely rebuilt the soundboard that doesn't work, so it's most likely something I did. The original board produces a little more buzz, but it also works 100% so I'm not complaining. I'll order some more capacitors and at least get that much of the old board rebuilt.
There are a couple of sounds (the fly-by sound when you launch and orbit and the match explosions) that didn't work before. It's like a new new game now!!!