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:
opening: this was an idea for the original Doom title screen song.
un17: Would have actually fit in the game.
un30: I like it's funkiness. Reminds me of Wolfy.
un36: I hated this song. I assigned it to e3m6 originally and everytime I ran that level I just cringed. I eventually removed it.
un39: Would have been a cool intermission screen.
un52: A better version of this song was in the game.
I think a few of the songs are early versions of some that were in the game. So for any of your Bobby Prince fans out there, here's more of the man's work.