Please provide the link to that video. It could well be that it's not supported by the app (even a smart app is a dumb browser), but that's impossible to say without knowing the video.
I have this issue with the no sound with the new PN51D6500. I was online looking at a video online using the smart tv internet app and can see the video but get no sound from my TV's internal speakers. Please let me know what I can do.