I know this thread is a year old, but I thought I should post my solution in case others with this problem come across this as I did.
I was having tis problem on both my laptops which I have connected to my living room and bedroom tv. Here are my equipment and connection profiles:
Asus G73jw with nVidia video card
Windows 8 consumer preview 64 bit
Onkyo tr-609 recevier
Samsung flat screen
Laptop connected to receiver via HDMI
Sony 2011 Z series
Docking station with an ATI video card that allows connection of multiple displays
Windows 7 pro 64 bit
Onkyo tr-509
Samsung flat screen
Laptop (not docking station) connected to receiver via HDMI
The solution turned out to be uninstalling all Realtek drivers and software, restarting the computer and letting Windows install it's own sound drivers. That cleared the problem. Somehow, whatever the realtek drivers pass through to the recever are not 5.1 surround or cannot be decoded.
If anyone wants more detail, post here and I will reply.