Hi. It is likely that this is the same as https://bugs.debian.org/697266 - linux-image-3.2.0-4-amd64: Please backport support for HDMI audio on recent RADEON graphics cards.
Long story short: the necessary PCI IDs for the HDMI audio on this card are not present in the kernel - it was added in kernel 3.3. If you are able and willing to install linux-image from wheezy-backports this should allow the HDMI audio to work. Otherwise if you build the kernel yourself with the patch attached to that bug report, it should also work. Cheers.
