Thanks for the tip.
I finished Kane and Lynch a few moments ago(after 5hrs...). I reinstalled the KX drivers and I tried your advice after that and it helped a little, but I hope this "bug" won't happen more often. By the way, K and L was the first game with this bug, so I think the game isn't innocent ;-)
But, I delete the game now anyway.
The problem with the lagging videos is more important for me now, because I wanna play now the other game (The Experiment) and don't wanna miss KX Drivers :-(.
In the forums on Gamespot, they reported, that the problem is caused by the sound driver. And when I installed the original driver, it was gone. What could be the reason for that? Thanks for your help :-)