Same problem here, 320GB Slim. Im using an Onkyo AV for the sound.
The only way to play with cutscene's voice is not updating. But i wanted to play coop the whole game.
I hope this is fixed asap.
Randy
PD: I understand now why some people were returning the game because failed sound. A bit extreme i guess but understandable.
btw I work on a game store.