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.