Thanks and sorry about this. The game is from the Pentium era, so 68040 is as low as it goes.
We are also back to AHI. It turns out there was a way to boost the volume back, using the
AHIET_MASTERVOLUME effect. It's still a bit quieter than it used to be, but this is as high as it can go. Many thanks to koobo for pointing me in the right direction!