Admittedly, there is a small split-second gap at the very end of the MP3 version that prevents it from looping perfectly. The OGG and FLAC versions seem to be unaffected. I'm not sure what to do with this, except joining the "bleh, MP3" choir in the back row right over there. Perhaps i am going to release an "album" version someday that will feature seamless looping, extended length and a proper fade-out. Someday.
Okay, so here is what I think happened. The mp3 format has a bit reservoir, and there's a buffer of bits that are unused by default. That's why true "gapless" playing is notorious, since there's always that spare bit capacity that needs to be flushed out. Depending on the audio player used, you may have an option of "gapless" playback that you can use to get the nice loop.
Simon-v, I highly recommend that you make two versions of your music, namely the "studio cut" and the "game version". We will use the "game version" in-game, and compile the "studio cut" stuff to make into a DoomRL album of some sort. I think it'll be an awesome idea; in fact, if you follow
MSPaintAdventures, you'll see that they have done something similar, releasing the tracks they used in the animations (and not).
If you are willing, maybe you can contribute some of the proceeds of the sale to ChaosForge itself too! =P