This is probably a common problem among all the people trying to get audio over HDMI under Linux using an EN210 graphics card (or similar).
Even though the HDMI may appear in your available cards listed in proc (/proc/asound/cards) you will probably see that you cannot find it using the aplay utility. Just check it running the command “aplay -l”. The HDMI device isn’t listed here isn’t it?
The reason for this issue is related to the ALSA driver version 1.0.21 (or older) which is quite buggy for these graphics cards.
You can check which version you are using by executing the following command:
If you get a version older than 1.0.21 then you need to upgrade it. You can check your favourite distribution’s repository for an update or you can try to compile it from source code.
If you can’t find an update, check this page: http://ubuntuforums.org/showthread.php?t=1681577
It might help you 😉