I'm not sure if you found my post already https://pinside.com/pinball/forum/topic/calling-all-williams-system-11-sound-gurus-having-volume-issues but i had the exact same issue. Pin-Bot was SCREAMING at me. And the explosions were ridiculous. I wired up a resistor and a POT to the input from the daughter board on the MPU to the sound board and it did the trick I actually cut the red wire i believe on that small harness and put it inline, it's in my thread). It looks kinda hacky, but it worked. Pin-Bot was SCREAMING at me. And the explosions were ridiculous.

When i contacted Rottendog about it, they send me a new chip for the daughter board and it did nothing. When i contacted them again they said they were out of ideas.

Also check your R32 resistor and see if it's a 2.2kohm or a 10Kohm, this is the first spot the incoming sound from the MPU daughter board hits. (this is also in my thread above). The parts list says it's supposed to be 10Kohm but i have a 2.2Kohm installed which is what the schematics say, so something is screwy there. My soundboard is an original Pin-Bot soundboard, and R32 looks untouched from what i can see so it's not a situation where someone changed it, or it's from another game. So i may try putting the 10Kohm in there and abandoning my external pot setup and see if that works.