No Audio via Optical cable for TV, Movies - only Music

Hi,

I have a 3rd Gen ATV with optical output. Optical cable is connected to my amp and HDMI cable to TV.

Sound plays fine via amp & speakers when I am play music and music videos from ATV, but doesn't play sound when I play TV shows or movies (picture plays fine and sound comes through TV speakers, but not through hi-fi speakers).

Optical cable obviously works ok as Music plays fine.

Any suggestions?

Many thanks

Toby