Sound output using optical connection on UE43NU7400

[BazzaM]

I use a Sky Box and an amplifier as sound outputs, with the volume on TV set to zero to avoid an echo.  This is fine when I watch TV programmes with the sound output set to either TV speaker or optical.  However, I get no sound when watching an app (putting the TV sound up plays through the TV speaker but only on the TV).  I have done the sound check and it plays through the amplifier.  Furthermore, it makes the normal beep sound when selecting an app.  This is a new problem; there didn’t use to be an issue.