Phoenix1: There seems to be an update already some weeks ago on Steam, which (among other things) fixes the annoying sound bug.

Here ist the text:

"Update July 28th 2017
Changelog
- added localization for easy/normal mode switching warnings.
- Fixed video playback issues.
- Fixed sound issues, missing music in some levels, some effects playing through left channel only or missing, etc..

Known issues:
- Music is playing low through rear channels in 5.1 and 7.1 setups. "

When it is coming to GOG?