Audio problem: At the end of the movie when they sing "you are my sunshine" they movie into a L.S. and you can see that the words aren't synched in to their lips. (01:31:30)
Audio problem: When Everett is hugging his three girls after they sang on the stage there is a horribly garbled part of the audio. This occurs on DVD and when it is broadcast.
Answer: Probably they have dismantled the coffer dam that was used while constructing the main dam. Hence, the flooding.
Good and reasonable last resort explanation - have the Coens ever addressed it?