User Manual:HDAudioPassthrough

Those are preliminary information on how to enable HD audio passthrough over HDMI (currenly E-AC3 and TrueHD).

You'll need a recent linux kernel (>= 2.6.36) that contains the required ALSA drivers. Alternatively get the latest ALSA driver from the git repository

git clone git://git.alsa-project.org/alsa-driver.git alsa-driver 
git clone git://git.alsa-project.org/alsa-kmirror.git alsa-kmirror 
cd alsa-driver
./gitcompile
sudo make install

Alsa library 1.0.23 is the required minimum. However if you had to mess with the device name to get basic audio working, it is strongly recommended you get a new version as otherwise you will likely have to mess with alsa configuration file.

git clone git://git.alsa-project.org/alsa-lib.git alsa-lib
cd alsa-lib
./git-compile
sudo make install