Comment 32 for bug 1020041

ATI/AMD Radeon HDMI audio has been disabled by default by the upstream kernel developers as it causes blank screens on a lot of systems. You can re-enable it by adding radeon.audio=1 to your GRUB boot options as described at https://help.ubuntu.com/community/Grub2/Troubleshooting#Editing_the_GRUB_2_Menu_During_Boot

For your HD 6000 series chip you will also need a 3.5 or later kernel. Ubuntu 12.10 "Quantal Quetzal" is based on the 3.5 kernel.