I cannot turn down the music volume from the in-game menu or from the setup.exe on One Unit Whole Blood. I had to turn up the sound effects in order to hear them in compairison to the music. Thanks for the help.
EDIT: Woops! There is already another post about this: http://www.gog.com/en/forum/blood_series/redbook_audio_volume ... My apologies.

it must be a bug. I removed the .ogg files so the game would switch to MIDI music and the music volume settings have no effect whatsoever, it keeps playing at max volume.

You can lower it in the Windows Volume Control panel by reducing CD Audio Volume.