She was prerecorded singing to the karaoke instrumental version of "Christmas, Why Can't I find you?" by ProSound Studio Band (instrumental version only) http://itunes.apple.com/us/album/chri... I then took three video takes of her actually singing to the recording of herself, cut the video and matched it up to her own recording. The prerecording was to get the best audio quality and the three takes were to change it up a bit with three different camera angles.
For those who don't believe it's her real voice can play it simultaneously with Cindy Lou's (Taylor Momsen) version at http://www.youtube.com/watch?v=tc7Xix... If you listen closely you'll notice they're two completely different voices by way of voice fluctuation, pitch and enunciation. Also, she sings "AND nothings easy" at 1:07. Cindy sings "NOW nothing's easy."