DVI doesn't output 7.1 channel sound. DVI doesn't output any sound. Just a minor difference between HDMI and DVI.
There are video cards with optical and HDMI output, but they're uncommon.
Anyway, nobody wants to keep a PC in the living room to use like a stereo component. It's ridiculous. I say this as someone who has a PC with optical/HDMI output, a TV tuner, AV input/capture, etc. etc. All that flexibility makes it useful as a presentation device in an auditorium or a meeting room, but the living room?
Some people complain about the size of the PS3 as a living room stereo component. Good luck coaxing the general public to use a PC tower for the same purpose.
Do you use a remote or do you use a keyboard and mouse/pad to watch TV or movies in the living room? Sure it can be done, but the overwhelming majority of the PC using world doesn't and doesn't want to either.
I do it myself, but it's awkward and impractical.
I'm not aware of too many output sources for 8 channel sound equipped PCs, but maybe that's just me.