I had my TV running on Optical only the other day, but I then heard a weird noise from the speakers, like a lot of white noise. I thought it was nothing much at the time.
However, I've noticed that if I try to play Amazon Prime or Samsung's TV streaming services etc through Optical out only, it doesn't work. Unless, that is, I switch to PCM only on the expert audio setting.
I've reset the TV, confirming that 'auto' is the default setting for Optical out, but is this a fault or is it just some wierd issue whereby you have to manually switch to PCM for Amazon and Samsung Streaming? Weirdly, YouTube works fine in auto mode.