I just got a hd6450 and when I connect hdmi to the TV I can get 2 channel sound over HDMI. However when I connect it to my Sony str-dn1030 I can get the video to show but cannot get it to play any sound, not 2 channel, not 5 channel and not 7 channel, I must be missing something but I cant figure it out.
I've just been having the same problem with only 2 channels appearing on my AMD HDMI device. The problem was being caused by my Onkyo receiver. I had HDMI passthrough enabled on the receiver. If I turned the receiver off before the TV, the PC only saw a 2 channel device (the TV) which is why the 7.1 option was missing. As soon as I disabled HDMI passthrough the 5.1 & 7.1 surround options appeared in the speaker setup.