Because OPL3 is not midi.

the sound formatting of how the midi track is structured/mastered is likely different to how the in-game tracker plays the adlib standard FM audio - i think there are more effects that midi can do that FM can't, which is probably why it sounds different.

Yes it depends on how midi commands are mapped to the OPL. The SB16 and Aztech sounding the same in Windows isn't even a given, they probably just use the same driver internally but a different sound card maker might decide to have his own midi->OPL conversion.

Obviously the game does things its own way, and there's even ways to change that: https://github.com/sneakernets/DMXOPL

Additionally a .mid file might not match a game's internal midi format, so even a GM device might sound different between playing a file and actually listening in-game. I think Doom files fine, but for example the midi files that come with System Shock on Steam are way off due to a flawed conversion process.

Oetker wrote on 2020-02-20, 07:47:

Additionally a .mid file might not match a game's internal midi format, so even a GM device might sound different between playing a file and actually listening in-game.

I read something similar about Duke3D as well.

Supposedly, the music sounds slightly different when played in-game.

Oetker wrote on 2020-02-20, 07:47:

Additionally a .mid file might not match a game's internal midi format, so even a GM device might sound different between playing a file and actually listening in-game. I think Doom files fine, but for example the midi files that come with System Shock on Steam are way off due to a flawed conversion process.

I actually used .mus files straight from the Doom .wad file.

Anyway, thanks for clearing that up, guys.