>Installed ubuntu 12.04 and I had no sounds coming out of my HDMI cable. This is an easy fix that I learned while using linux Mint. Edit the grub.cfg file and add the option
more specifically edit the file /boot/grub/grub.cfg and find the menu entry you want to use and change the linux line. ie.
linux XXX ro quiet splash radeon.audio=1
I removed the XXX's because they're kernel, and HD specific.
The next step is to make this a permanent change, and have it applied to all of your kernels even if you update.
Edit the file /etc/default/grub , find the line:
And add the radeon.audio argument to it.
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.audio=1"
You could use the GRUB_CMDLINE_LINUX if you like, but the DEFAULT will only be used with the normal boot, and not recovery.