TomTom:
We're using WMV videos so installing extra codecs is not necessary. They should run out of the box with up to date Windows installations.
If you're playing on Vista, make sure DirectX9.0c is installed. Also, during installation, PhysX drivers are installed, and Visual Studio redistributables. These are all shipped with the game, and are installed by default, but maybe during install you canceled it or unchecked the box, or something went wrong installing them. You should be able to find them manually on the DVD and install them directly without having to install the game again.
It might be that your Creative X-Fi is causing the problem. Try updating its drivers, or if you have an onboard sound card, disable the Creative X-Fi and try playing with that onboard soundchip.
(Sorry for the English reply, I can read German, but not write it :))
