part of it too is the timing of the video record on the camera vs the timing of playing the mp3 file on the computer.
Windows used to use a streaming clock proxy (not sure if it's still used.) A stumbled upon this when I had a client that would have mp3's play back sounding like the chipmunks were singing the song no matter what program you used to play the file. After replacing the damaged file, then everything started playing normal again.