AMD has finally published NDA-free public programming documentation concerning the HDA audio support on Radeon GPUs.
Released on Friday was the 10 page PDF file
yielding new light on the audio support built into modern Radeon GPUs. Up to this point, most of the Radeon GPU audio support was implemented via reverse-engineering the proprietary Catalyst graphics driver
Radeon HDMI audio support is still disabled by default in the Radeon DRM driver due to some bugs, but those might be worked out in the latest kernel update and lead it to being turned on by default in the near future
Other good news with regard to the open-source Radeon driver can be found in this forum thread