1. Upgrade to kernel 3.12, possibly better 3.13RC, there were some issues with HDMI audio in earlier kernels. You should use a 3.12 kernel anyways to get power-management of your APU fully working.
2. As you have already done, add the radeon.audio=1 parameter to the kernel command-line (and radeon.dpm=1 to activate dynamic power-management).
3. After booting try to enable HDMI audio:
xrandr --output HDMI-0 --set audio auto