I've been trying to use the mt32emu device, but it isn't available. Looking at the commit history, it looks like it got patched out when the DOSBox r4250 was merged, but I can't tell if that was intentional.

I have started a branch (with the intention of doing a pull request) to try and re-enable the device adding the mt32 entry back in dosbox.cpp and the include block back into midi.h.

#define DOSBOX_MIDI_H
#include "midi_mt32.cpp"
static MidiHandler_mt32 &Midi_mt32 = MidiHandler_mt32::GetInstance();
#undef DOSBOX_MIDI_H

While this does seem to enable the device, it does seem to cause the app to hang and the fail to finish initializing (i.e. you never get to the prompt). My Objective C/Swift is... lacking.