As Asmodan says, they are just .ogg music files. For what it's worth, all of the music that I have personally put up there is made freely available from other sources. For instance, the Portal 2 music is freely available from
http://www.thinkwithportals.com/music.php. There's another thread on these forums (do a search for it) where I try to post whenever I come across some cool free music. The only thing I have done with these tracks is to convert them to .ogg (if they were in another format) and standardize their filenames to ARTIST - Track Number - Track Name.
Another thing to note, you can put any song you like in the music folder. If you're sick of hearing Level6.ogg, you can take any .ogg music file, rename it to Level6.ogg, and stick it into your UT2004/Music folder as Level6.ogg. I recommend doing this for NanoClean.