Had the same problem on Linux Mint 14. The following solved this issue for me:
To setup the HDMI sound system for output get the package - pavucontrol
In the terminal, first just type: pavucontrol
If this app does not exist then get the app by typing: sudo apt-get install pavucontrol
followed by your password.
Now run this app again by typing: pavucontrol
When the app launches it should say Volume control at the title-bar of the app. Select the configure tab / from the drop-down menu for profile - select the HDMI option.
On the Output device tab, the port should read HDMI....
That should do it!!! It worked for me and I hope you are lucky as well.