JW McNay (MurderHobo's music composer) remixed all the game's music in Chiptune. What's Chiptune, you might ask? I think it's a species of tree-dwelling Japanese rodent. They have a magical power that makes things super nostalgic. The cave theme is giving me flashbacks of playing Platoon on the NES as a kid, only the fun parts and not the invisible tripwires.
You can download the alternate soundtrack from here:
To hear it in-game, unzip and copy the files to Desura/Common/MHAQ/sound/bgm (if you're using a Desura client install) or MurderHobo/sound/bgm (for a standalone install).