If you are have a old pc running a lightweight linux interface like LXDE or XFCE, e.g LUbuntu and need to get the audio out to work via the HDMI you most probably have notices there is not a nice GUI to do this like in Ubuntu.
pavucontrol is a great and simple solution. I know it is a bit heavyweight since you have to install pulseaudio but it is the easiest solution I have found. To install:
sudo apt-get install pulseaudio pavucontrolthen to run