This guide will help you fix the issue where the game Killing Floor 2 is muted when you use steam overlay, alt tab, or whenever the game is not the focus.
Fixing Losing Sound (E.G: Alt+Tab, Steam Overlay)
Navigate to KFGame.ini file Located as shown below in the picture
After that open the file and search for the word "mute" Ctrl + F then type mute:
Change the line from True to false. Done!