I tried a couple of things without success:
1) adjusting musicvolume entries in outcast.ini. However the game overwrites outcast.ini each time it runs so the changes don't stick. Changed outcast.ini properties to read-only but game still overwrites.
2) more drastically - looked at changing the sound levels of the music files themselves eg using Audacity or some such. However they're packed in some obscure old format (sonarc) that I can't get at.
I too would be interested in any ideas to get the music volume down but so far I'm drawing a blank. Will probably go back to my CD version in the meantime, if I can get the patches to work...
I guess if you just wanted to disable the music altogether you could try renaming/removing the music files.
Wavell