Comment 7 for bug 1259423

I noticed in another bug report that you want to use fglrx because you can't get HDMI sound with the open-source driver.

On kernels prior to 3.13 you can enable Radeon HDMI audio by adding radeon.audio=1 to your GRUB kernel boot options as described at https://help.ubuntu.com/community/Grub2/Troubleshooting#Editing_the_GRUB_2_Menu_During_Boot

With the 3.13 kernel the radeon.audio=1 kernel boot option is no longer necessary. Instead, HDMI audio on ATI/AMD Radeon cards can be enabled and disabled on the fly by typing the following in a terminal window:

xrandr --output HDMI-0 --set audio on

or

xrandr --output HDMI-0 --set audio off

If your HDMI port is numbered differently (type "xrandr" to check) - you might need to change HDMI-0 in the above command to HDMI-1 or HDMI-2.

Ubuntu 14.04 "Trusty Tahr" currently has the 3.13.0-12.32 kernel. If you want to try and install the 3.13 kernel on an earlier Ubuntu release, a PPA is available at http://kernel.ubuntu.com/~kernel-ppa/mainline/ and instructions on how to install and uninstall it are available at https://wiki.ubuntu.com/Kernel/MainlineBuilds