No sound on HDMI with Radeon driver

My PC has a build-in VT1828S chip. The connection between PC and monitor (which has built-in speakers) is through an HDMI cable.

Audio preferences shows 2 devices for audio output: analog stereo and RS880 Audio Device [Radeon HD 4200] digital stereo (HDMI).

When I plug a headphone in the rear jack and I choose the former device, everything works. When I choose the latter device, instead, no sound is heard, and video playback is too fast. It is as if video frames want to synchronise with the audio timing, but the audio timing information is wrong.

HDMI audio worked perfectly with Ubuntu 11.04, but stopped after 11.10 upgrade.

HDMI Audio is disabled by default on kernel 3.0+ due to a black screen on some monitors to get audio working again:

1. Go to the terminal and write:

   gksudo gedit /etc/default/grub
   

2. Then change the line:

   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
   

   to

   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.audio=1"
   

3. Type on the terminal:

   sudo update-grub
   

4. Reboot and your HDMI sound will work again.

Here's the thread where I found the solution.