Aside from Radeon KMS HDMI audio not being enabled by default, HDMI audio support has entirely been missing for newer Radeon GPUs, like the HD 7000 "Southern Islands" hardware. Fortunately, it's being worked on and will hopefully land with the Radeon changes for Linux 3.12.
On Tuesday, AMD's Alex Deucher published a revised patch for enabling audio support on DCE6/DCE8 GPUs.
The DCE6 display core has been around since the Radeon HD 7000 series while the DCE8 display core comes with the AMD "Kaveri" APU, the Radeon HD 7790 "Bonaire", and AMD "Kabini" APUs. The display hardware is different than earlier generations with adding HDMI 4K display support, VGA/LVDS via DisplayPort bridge hardware, etc. Alex notes in his patch that the audio support for DCE6/DCE8 is similar to DCE4/DCE5 hardware but there's support for multiple audio pins.