Contract sound designer, musician for Tripwire Interactive. Seven plus years working on Killing Floor. Various mods/games over the past eleven years.
I've written a series of in depth tutorials for the musician working in the UT3 mod community. The tutorials include a start to finish import guide, how the engine chooses which music segments to use at which times, and how level designers can add music, in whole or partially or mix-and-match, to maps without modifying the original files.
These can all be found here.