OPL Music Emulation
Chocolate Doom recently implemented OPL Music Emulation. While it isn't perfect, it could potentially help Odamex for two reasons:
1) The cool reason. OPL Music is cool, sounds "classic", and is a great feature to have for a Doom engine. It also fits within Odamex's scope of providing an online experience relatively close to vanilla.
2) The practical reason. With all of the midi issues sdl_mixer causes for Windows Vista/7 users, offering the OPL alternative is a great solution that can be done without any changes to SDL_Mixer. The more people upgrade to the newer Windows that lack proper midi mapper support, the more complaints Odamex and other SDL Doom engines are going to hear. Offering OPL music would potentially fix a lot of these complaints.
Chocolate Doom 1.5.0 was recently released and features a switch to DosBox's OPL emulator. The OPL Emulation seems to be separated in the code so maybe it's worth a look.
*** Bug 1161 has been marked as a duplicate of this bug. ***
Team Eternity recently implemented libADLMIDI into their Eternity Engine source port and it sounds amazing: https://github.com/team-eternity/eternity/tree/libADLMIDI-branch