See
http://www.alsa-project.org/alsa-doc/ . You'll be looking at using a mixer (alsamixer) and I believe you'll be wanting to play with the " unmute "IEC598 0"" output to hear anything. Also, it seems your results will vary with what a 'soundcard' will output on that SPDIF and what the 'receiver' will accept as far as format goes. Good luck.