I've been doing some digging around in the original Doom development directory since I'm working on consolidating all my data in a sane arrangement. The Doom source that was released years ago wasn't the nice raw development directory otherwise you would have all seen the NeXTSTEP DoomEd source, Doom map source files, and what I have here: unreleased Doom MIDI files.
There's a reason these weren't released - they're not very good, at least not as great as the music we actually shipped with the game. Some of the songs are just repetitive riffs. All the songs are named unXX.mid where XX is a number.
Some of the standout tracks are:
I think a few of the songs are early versions of some that were in the game.