Movie to short, Song to long
I do not know the song that well, the first time I heard it was in a flash on Newgrounds. I must say that it sounds like tried to make a round of it like row, row, row your boat.
Also if you want to use the whole song do not loop your animation please. Extend one or truncate the other instead of repeating it like you did.
If the sond started over with each repetition that could explain what I was hearing if I wasnt imagining it.