surround sound through hdmi issues

I have just sent up XBMC 12.0 “Frodo on my desktop and have set my audio to Tru Hd / Dolby Digital enabled with HDMI selected. But when i go to play my files the movie has no sound and the picture is slow. It will work when i change HDMI to analog. Can someone please help.

I currently have a VTX3D AMD Radeon HD 6670 2GB GDDR3 PCI-E Graphics Card which im not sure if it has 7.1 channel support.