I am built a serial MIDI sound module with Seeed XIAO MIDI Synthesizer and it compatible with SoftMPU.
Since XIAO MIDI Synthesizer is just $20, it might be the cheapest MIDI synth work with SoftMPU.
https://www.seeedstudio.com/XIAO-MIDI-Synthes … zer-p-6462.html
Also itss ESP32-C3 MCU could be programmed by Arduino IDE to accept all RS232 baud, so it could use with DOSMid serial mode (19200bps), SoftMPU(38400bps) as well as other serial MIDI applications
https://wiki.seeedstudio.com/xiao_midi_synthesizer/

My Arduino sketch was slightly modified from diyelectromusic's, which forward MIDI commands from both USB serial as well as TTL serial defined on Pin 9/10, so we can connect it to DOS computers with any cheap RS232-TTL adapter
https://gist.github.com/pengan1987/24ca07184e … faa8df79a5a9eb4

For SoftMPU, it could be used in serial mode like

1softmpu /mpu:330 /output:com1

I tested with Silpheed, Dune II, Prince of Persia with SoftMPU, all works fine with my Pocket 386, check video on: https://www.youtube.com/watch?v=l268lij5HH0

For DOSMid, it could work either with SoftMPU or directly to serial, but since DOSMid use MODE command in DOS to set baudrate, the highest baud is limited to 19200, and have to change MySerial1 baud in arduino sketch to 19200

1// MySerial1 is the TTL serial on pin 9/10
2// Change MySerial1 baud to 19200 for DOSMid
3// MySerial1.begin(38400, SERIAL_8N1, 9, 10);
4MySerial1.begin(19200, SERIAL_8N1, 9, 10);

And then you can play MIDI files without SoftMPU

1MODE COM1:19200,N,8,1
2dosmid/com1 music.mid

Since XIAO MIDI Synthesizer also accept USB serial input, it also working toghter with Hairless MIDI<->Serial Bridge for modern computers.
https://projectgus.github.io/hairless-midiserial/