The cast album only has a few moments of her singing, since the role of the younger sister is a fun, comic part, though not one that calls for songs to move the plot along.
That makes this video find all the more a treat. It's a 3-1/2 minute scene that has a very young Anna Kendrick in the center -- and ending with a duet sung with Melissa Errico as her older sister, Tracy.
If it looks like she is wildly overacting -- she is, and it's intentional. In the scene, Tracy learns that a couple of reporters have been invited to come to the family's palatial estate to cover her wedding. She hates the idea, sure that the journalists are there simply to ridicule the event, so comes up with a plan to give the reporters exactly what they want -- and she and her younger sister pretend to be beyond ultra-pretentious.
Keep in mind too that though she looks like she's eight-years-old, Anna Kendrick is 12 here.