The problem is probably the program you are using to play the video.
Try using different video players.
I recently purchased a canon rebel T2i. I took an important video with the camera and want to upload it to my computer. It works great on the camera but when I upload it, the sound and picture are badly out of sync. Is there anyway to fix this or a specific way to upload that works better than others? How do I prevent this in the future?
Thanks for any advice.