When starting a game there will be light flickering around you in a shape of a square.
And the audio will go missing. Not all of it but things like pick-up sound, reload sound, gunshot sounds will go missing but will still retain zombie audio.

Boot up Dead Center. Once you spawn in you will notice the door and props flickering and the audio missing for certain actions. Right now I have only this mod disabled on my main modlist and the issues seems to be tied to it.