sound output problem

[Banny]

Have my Shuttle hooked up to an AV reciever via optical out and have a sound issue.
If I play MKVs I get sound fine, anything else no sound so none in AVI's or MP3s is there something I am missing do I have to use the other sound outputs to get the missing sounds.
All is fine in DTS stuff but nothing else.
Thanks for any help.

 

[Banny]

where am I going to plug the optical into.

 

[Banny]

Thanks have managed sound but no sub working either with the optical or the single connection in the green socket.
Its weird now as my ipod is not giving me the sub, but it did before I messed around with the pc settings.