hi there! for me, the audio issue indeed turned out to be a lack of a built-in midi engine on linux. I installed the flatpak for Qsynth, went into Setup->Audio->Driver=Alsa and added the default windows soundfont I downloaded somewhere, and MIDI works everywhere on my machine now. someone else on discord didn't have this work for them though, so let me know how it goes for you.
for EasyRPG at least, it has a built-in MIDI engine but might need a soundfont. you can try running it through terminal and see what errors it spits at you, as a different option from installing the general-purpose MIDI engine.
good luck!!