Audio problem: When the carolers come to sing for the Kranks, it is professional-quality singing. Then the neighbors join and you can hear that more people are singing, and the quality of the singing is realistically worse(the neighbors can't sing well.) After a few shots of the carolers AND the neighbors singing, the sound goes back to just the carolers singing, though you still see the neighbors singing on screen.
Audio problem: Towards the end during the party we see a guy playing some sort of small pipe instrument. First of all, the timing of the moving of his lips on the instrument doesn't match the pipes heard in the music. Secondly, the lower notes that are heard in the music are heard when his mouth is at the shorter end of the pipe, which would realistically result in high-pitch sounds.