The latest update has broken the game on Manjaro/Linux. The audio is gone and I can't even interact with something. The windows version with proton works so far. But I bought the game because of the Linux version.

Just in case you did not resolve this by now, the problem is the "too up to date" distribution of yours;
as 'Player.log' says: "Fallback handler could not load library /home/'username'/Spiele/GOG/Slender Threads/game/SlenderThreads_Data/Mono/libfmodstudio.so"

see here
which links here
The solution, as mentioned in the itch.io thread, is running "execstack -c (your game installation folder)/game/SlenderThreads_Data/Plugins/libfmodstudio.so"