Lars-Göran Nilsson wrote several articles on the sound capability on the R600's. He tries to resolve a "mistery" (for him at least), how the R600 cards send audio signals out on the DVI connector (that a DVI->HDMI adaptor forwards on the HDMI output). He is guessing there is a "proprietary DVI connector" on the cards.
This is unecessary. To understand how it's done, we only need to take a brief look on the pinout and specs of the HDMI connector, as well the DVI. HDMI, DVI There is no separate audio lines on HDMI! There is only the same TMDS data lines, that are there on the DVI (regarding the high-bandwidth lines) -- just there is only one "link" (but which in Ver1.3 HDMI can be 2x the speed of DVI's, per link), instead of two on the DVI (the second one used by a dual-link connection only).
You may ask, how the HDMI transports the audio signals, then. Well, by putting them into packets, that travel along with the video packets on the same link... And it holds the solution to this "mistery", as well. So, when you go for HDMI with your R6xx card, by plugging in that DVI->HDMI adaptor, the card simply sends the audio packets along with the video packets, on the first link, that the adaptor simply forwards on the HDMI connector.
That's all about it.
ps. I heard the R6xx's are capable of Ver1.3 HDMI (like higher transmission speed), so resolutions above 1920x1080. It can be done only on dual-link DVI, but as we talked about it, HDMI is one-linked. There is a solution for this, too: in case of "doing HDMI", the card sends high-speed one-link signals on the DVI connector, that the adaptor forwards to the one-linked HDMI.