I heard this a while ago but I don't think I shared it.

http://www.youtube.com/watch?v=E-29NvPMYXI
(There are various services out there that let you download YouTube music, so you can grab it with one of those if you want to.)

This song sounds similar to "Too Hot" - http://www.youtube.com/watch?v=g5wyIQ6Yz4U - but obviously higher quality. So it may be a fan-made remake or recorded with a different sound emulator or option in DOSBox (however that works).

That box cover showing a dude with a gun always cracks me up. Like those lousy NES boxcovers everyone makes fun of, but more obscure.

I somewhat suspect the remix might have been done as a Commander Keen remix, since episode IV used the track as well, and it probably got more exposure through that game than Catacomb 3D.