In case the MT32 emulator author never fixes this, do the different audio software components go through a common mixer that can boost the volume of a particular incoming audio input such as the mt32 output?
But that mixer is based per type (music, voice, sfx) and not device. We need to boost the volume base on device, which in this case is the mt32 emulator. I guess I can check out the windows driver version of the mt32 emulator, have scummvm use the windows mt32 as an external midi device, and boost the mt32 volume from windows.