Fix for Linux native version!

[djcj]

On Linux the sound is off and the menu is partially broken.

I figured out you can fix this by going into Steam -> Right click on DeadCore -> Properties -> Compatibility and there you select Steam Linux Runtime 1.0 (scout).

So it's apparently an issue related to newer Steam runtime or native Linux runtimes.
Of course alternatively you can also play the Windows version by enabling Proton, whatever works best.