russellskanne: I think I wasn't clear enough. Off course I can move the slider and switch the mute button, but it has no effect. Though the settings get saved and I can also see them in the config file (GameUserSettings.ini):
...
AudioSettings=MasterSoundsVolume=1.000000,MusicSoundsVolume=0.000000,bIsMusicSoundsOn=False,SfxSoundsVolume=1. 000000,bIsSfxSoundsOn=True)
...
I've done a little testing and found out that it works correctly with patch 1.05b but NOT with 1.05c. The master and sfx sliders and mute-switches work, but the music ones don't.
So it's a bug that was introduced with the latest (gog)-patch.
You are absolutely right! Thank you for reporting the issue. I will forward it immediately.
As a temporary solution, I guess you could mute the game in your headphone's settings.