Stubborn game, doesn't seem to have a setup for choosing settings, rather decides to do everything itself
The manual isn't any help either.
Diable the PC speaker emulation altogether in dosbox.conf maybe(pcspeaker=false)? Try disabling other sound stuff also, maybe that would force to game to look for the MIDI output.
Should probably shell out a couple of bucks and look for a real, used MT-32 on ebay or something myself. Tried emulators but looking at youtube recordings of the real thing they aren't the same.