I know #LinuxMint 20 is due to be released soon, which will obviate this issue, but today I figured out why 19.X versions based on Ubuntu 18.04 don't play MIDI files correctly. Turns out Fluidsynth has a bug (332) which results in it being unable to find the default soundfont. Since the default media player Rhythmbox relies on GStreamer, and GStreamer uses Fluidsynth to play MIDI files, this propogates down the dependency chain. This bug was fixed a while ago, but the version of Fluidsynth available on 18.04-derived platforms is rather old. The fix is to grab a newer Fluidsynth and its associated dependencies from the eoan or focal repositories.