Looks like its official; cut and paste from Lightbox:
"Some Samsung Smart TVs are currently experiencing an issue which impacts the sound working properly on video on demand apps. Samsung are working to fix this as a high priority as it is affecting all apps and not just the Lightbox app. This isn't affecting the other Smart TV's or compatible devices."
My new TV is a UA40 J6200. Lightbox, 3Now etc are unwatchable due to the lip sync issues. Does not seem to affect the Youtube app though.
Be aware if you are in the market for a new TV, might pay to wait for a bit, or choose another brand.