I picked up an Earthshaker the other day, and noticed the voices were much softer than the music and sound effects - I previously had an ES that I'd swapped my Pinbot board into temporarily that had the same issue a few years ago, so that's the first place I looked. When I checked the code on the sticker on the MPU, it turn it originally came out of an F14. Does this require me to populate the sound section in the upper left portion of the MPU, or is there something else I should try?

That's what I assumed as well about the amp section, but it's curious that the exact same thing happened with a different ES with a different 11A MPU. When I put the 11B in my Pinbot previously, Pinbot's voice was way louder than it normally would have been with the original 11A board.

Fixed the issue - I guess I was always under the impression that there would be no issues when using a system 11A with an 11B game, and maybe it's just more noticeable in ES, but I learned quite a bit in the last 24 hrs or so, and wanted to share my findings just in case it helps someone along the way. PS - Sorry if this is covered in another thread, I just hadn't come across one, and the more the merrier.

Super close by both responses above -

I looked at schematics, and it looks like sound/speech mixer section of the mpu has a different set of resistors and caps for 11B vs 11A (consistently through several examples of both 11A and B). I haven't picked up the caps yet, but I got a perfect mix with an 11A MPU in an 11B game with the following resistors swapped out.

R21 needs to be 12K in 11B, and 15K in 11A
R22 needs to be 22K in 11B, and 27K in 11A
R19 needs to be 56K in 11B, and 27K in 11A
R16 needs to be 180K in 11B, and 56K in 11A
R17 needs to be 270K in 11B, and 220K in 11A
R15 needs to be 180K in 11B, and 43K in 11A
R20 needs to be 12K in 11B, and 27K in 11A

The following caps are different as well (although I haven't personally swapped them yet)

C12 needs to be 470pf in 11B, and 1200pf in 11A
C11 needs to be 100pf in 11B, and 180pf in 11A
C10 needs to be 470pf in 11B, and 1800pf in 11A

The pics in the previous post are exactly of that area on the MPU.

Anyway, it worked for me, hope it helps someone else out as well.

Just swapped out the caps, and everything is perfect. Before doing so, the voices seemed a bit grainy - It's perfect now from what I can tell.

Please do!