Radeon HDMI audio support in the open-source Linux GPU driver dates back to five years ago when the RadeonHD DDX driver gained reverse-engineered HDMI audio support. This early reverse-engineered support in the pre-KMS days was written by Christian König who later joined AMD as part of their growing open-source team.
The HDMI audio support was later ported to the Radeon KMS/DRM driver and merged into the 2.6.33 kernel. HDMI audio support came later for newer GPUs and took a long time for the HD 6000 series.
While it was merged, the HDMI audio support ultimately was disabled by default. Using audio over HDMI on Radeon graphics cards currently requires setting the radeon.audio=1 kernel module parameter on any recent Linux kernel release. The support was disabled by default since for some Linux users, having this support enabled would cause issues like blank screens when booting a kernel.
As reported earlier this year, Radeon DRM driver HDMI audio improvements were made. Rafał Miłecki made driver changes to mirror the HDMI handling by the open-source driver in the same way that the proprietary Catalyst driver is handling HDMI. With the driver handling HDMI in the same wasy as Catalyst, the problems went away on his hardware. There's also been other HDMI-related Radeon DRM improvements made in recent kernel releases.
In responding to a Phoronix reader question in our forums, AMD's Alex Deucher wrote today, "There were a number of monitors that would display blank screens with hdmi audio enabled by default. We fixed a number of bugs in the hdmi packet generation in the last couple of kernels so if all goes well in this cycle we can probably enable it."
So if all goes well for the Linux 3.11 kernel cycle, it looks like AMD may re-enable "out of the box" HDMI audio support, which could then happen for Linux 3.12. For now, Radeon HDMI audio users will need to keep set radeon.audio=1 for having this basic but important feature.