Check out http://forum.linuxmce.org/index.php?topic=2921.0
for similar issues, and the steps that were asked for resolution.
Steps I followed.. goto LinuxMCE Forums -> LinuxMCE, Fill in AC3 Passthru in the Search window and click Search. This searches all sub forums for that string anywhere in the postings. Then you can try SPDif or Audio.
Your question actually helped me out a bit.. Gave me some more things to try with my audio problems, different search string