So in the beginning everything went fine, but after Geron and Nuri leave Andergast the sound suddenly vanished and only reappeared in some scenes. Then after the cut scene with Nuri fighting in the wagon the screen just stays black and I have to kill the game.

Any ideas what to do about this?

I can't even start a new game now as cut scenes won't play at all now and the sound is gone from the start.

Okay, seems like my PC won't play any videos now after this happened, not even on youtube o.O