It only covers the original game and not any custom (non-commercial) addons that came with their own music, though. It's still great that this project exists to provide a fair chance for everyone to listen to Duke3D music the way it was meant to be heard. It's just to point out there is a decent (and free) alternative even if you don't want to rely on soundfonts or pre-recorded files. I am not saying that this is better than the SC-55, that's up to individual taste or preference. If anyone wants to know what the Yamaha S-YXG50 software synth sounds like, here are a few examples (not the best quality, though): I am just not playing enough old games with MIDI music these days to justify the effort.
Actually I own a real SC-55 unit (not a Mk-II, though) and a DB50XG daughterboard which I can connect to a modern system via a Doxbox, but tbh it's not worth doing it just to play Duke3D and Doom occasionally.
I am aware of the existence of that SC-55 plugin.