i post a few youtube videos n i have seen the same problem in several instances on different cameras. i have a middle-range sony that the sound was 3-4 seconds behind the video. I formatted the memory and all was cured.
at first i thought the video codecs in the editor were screwed up but when i played the video on my camera it was off there also.
if you have a lot of pics / movies, i would copy them to my computer n then delete all of them. not a guarantee but may help.