While HDMI audio is important to many users, especially when it comes to HTPCs, the support within the open-source Radeon DRM driver has had the feature disabled by default. It's been disabled by default for a long time now since for some Linux users having the support enabled has led to screen issues.
Enabling the Radeon HDMI audio support has required setting a kernel parameter of radeon.audio=1 to enable the feature. The good news is that the open-source driver's HDMI support might be fixed up where soon it could be flipped back on by default.
Rafał Miłecki posted a set of six new patches to the dri-devel mailing list on Saturday that clean things up a bit for the HDMI handling. Namely, these HDMI improvements make the open-source driver mimic the handling of HDMI to that of the proprietary Catalyst driver, where the HDMI audio support generally works favorably. Rafał Miłecki now things -- according to his tests on a Radeon HD 6970M and Radeon HD 6320 -- that the HDMI setup is mostly matched to that of the Catalyst / fglrx driver.
Miłecki ended his patch announcement with, "I hope that with such improvements we will finally able to enable audio by default."
For those curious about his reverse-engineering process of the Catalyst driver for analyzing the HDMI setup routines, Miłecki has written about his AMD Catalyst driver reverse-engineering on his blog.