mt32-pi is a bare-metal kernel that turns a Raspberry Pi 3 or later into a Roland MT-32 emulator (via Munt) and a SoundFont synthesizer (via FluidSynth). MT-32 mode covers early-90s DOS games written for the real hardware; SoundFont mode covers later General MIDI and Sound Canvas titles — between the two, it spans most of the era of DOS games with real MIDI soundtracks.
It natively supports GPIO MIDI interfaces, which is what makes PI-MIDI work: the HAT handles the MIDI and audio I/O, mt32-pi does the actual synthesis, and together they’re a small, cheap, authentic-sounding stand-in for hardware that’s expensive and increasingly hard to find in good condition.