Revealing mistake: Throughout the dance scene between Darcy and Elizabeth, a small earpiece can be seen in Elizabeth's ear. I would assume this is to help with the dance steps. This can also be seen during the dance with Mr. Collins, however it is not as noticeable. (00:39:00)
Revealing mistake: On many occasions throughout the film, Keira Knightley's real, short hair can be seen under her wig.
Answer: The composer is called Dario Marinelli, the pianist is Jean-Yves Thibaudet. Marinelli composed the music that Lizzy and Georgiana play. The only thing he hasn't composed is the piece that Darcy and Lizzy dance to at the Netherfield Ball, which is a piece from Henry Purcell.