The music seems to be overwhelming the audio for me. I'd like to turn the music down and leave the sound effects and voices where they are.
I know that the game has a "Balance" option, which is how you used to handle it, but I suspect it basically adjusted the CD-In volume slider in the mixer (which is irrelevant for the GOG MP3-based music). I tried the balance slider anyway, to no effect.
Anyone have any ideas how to turn down the music volume? I could always run MP3Gain on the music files and make them quieter, but it would be nice if there was some other way to get there.

Just to say I'm also finding this really annoying and would really like it if someone could come up with a solution. I'm sure The Moscow Symphony orchestra is a fine ensemble, and it's a famously good soundtrack, but it swamps everything else. It's ironic really, as the thing that kept me from playing this game again was the fact that I could never get the music to work. It was brilliant to see Outcast on GOG in a format where I could finally play it music and all, but now I just feel like shouting MOSCOW SYMPHONY ORCHESTRA SHUT UP!
I've tried playing with the music settings (MusicVolumeMin, MusicVolumeMax etc) in the various ini files to no avail. The changes I made to outcast.ini didn't stick, even when I've made it read-only... Made changes to soundcard_other.ini, they weren't overwritten but had no effect on the sound balance.

Try editing the OUTCAST.INI file in the install directory, changing the MusicVolumeDuringDialog and similar values in the [Audio] section to lower decimal percentages. Results may vary depending on OS and drivers.

My solution was not consistent , sorry.