I've had the same problem - no audio when playing a DVD with Windows Media Center on a PC.
I have the following units connected with a HDMI cable:
Samsung LN40A530 HDTV
Gateway p6830 fx laptop, Vista
Watched a downloaded movie last night with sound using itunes. Windows sounds also come across to the TV fine.
However, a DVD movie running on the pc (using windows media center) does not have sound on the TV.