So far the best places I've been able to find music files to edit into maps are these:

http://www.laernu.com/unrealfiles/Music/
Don't download the ones that start with 'ARC'. Whoever made those shamelessly promoted himself into the music.

http://sektornelsona.no-ip.org:9000/index.php?dir=Music/
Gotta be careful with this last one. You can only download 1 file at a time. If you try for more...well, let's just say it would be bad.

*ahem*

I've googled without a lot of luck, and checked the other places I usually get ut99 maps & skins & stuff without much luck.

What're some other good sources for .umx files?