The Game is great so far, but its all Stereosound. Other Games like Modern Warfare 2 or the Mafia 2 Demo are working in 5.1 - but not KAL2. What can i do to play the Game in 5.1?

My specs:

AMD Athlon 64 X2 Dualcore 5000+
PNY Geforce 9600 GT (OC)
4gb Ram
Realtek High Definition Audio