DVI does not carry audio like HDMI. DVI is a video-only format.
That is what I always thought too. But I have my GTX 570 connected from its DVI output to my 1080p HDTV's HDMI port using a cable with DVI on one end an HDMI on the other. I just switched Windows 7's default from my sound card/speakers to my Samsung TV. I am now listening to music from my PC through my TV speakers. I'm baffled, but it is working.