rtfm a little - although the fix causes me other issues as everything now goes though the spdif so i cant control the volume on 2ch stuff
-ao alsa:device=spdif -afm hwac3
or if that causes (like on mine) a high piched screech try
-ao alsa:device=spdif -ac hwac3
-ao alsa:device=spdif -ac hwdts
bit of a pain in the ass to have to change it all the time but that's the only way i can get it to play with ac3/dts under mplayer whether its though mythtv or mplayer