Here's the latest Doom for DC. After messing with the music for a whole day, I decided to take another approach - I converted libWildMidi to the DC and used it and mus2mid from Chocolate Doom to play the music. Funny enough, things sound rather similar... I wonder if my TIMM has some problems, or if it's the Dreamcast having the problem. I need to put some MP3s on a CDR and try it on the my system.
Anywho, give this a try. It's got a nice set of instruments on it - the same ones I use on my PC for MIDI music. The archive of libWildMidi is also attached here in case you wish to use it yourself. Just copy the lib and includes from the arc into kos-ports, add libWildWidi to the list in the makefile in kos-ports, then make. To see how to use it, look at the Doom source code in dc-sounds.c.