I have also experienced this with shredder pistol. Audio will cut (music, mobs, etc.) except for the sound of fall shells. Then when reloading audio will abruptly come back in.
I am having the same issue here. Never had this issue before, but today i changed my headphones cause the ones i was using died and now i have this twin-linked stubber bug. It fires 2 shots and the sound cuts out and only the falling shells sound remains. When i stop firing the audio returns to normal after a couple seconds and then it bugs out again every time i start firing. I triend on the ripper gun and on a bolter and the audio is fine, just the stubbers are bugged.