In the movie Swing Girls (which I briefly mentioned a few days ago), the actresses (and one actor) had been taught how to play music for several months prior to filming; as a result, all of the performances in the movie were "real," performed by the actors themselves. When the film was released, they did at least one promotional concert, entitled "Swing Girls' First & Last Concert" (although I've read on IMDB that several concerts were performed in both the U.S. and Japan). This video is of Glenn Miller's 1939 classic, In the Mood.
An interesting side note: In the Mood was apparently based at least in part on the melody of another song called Tar Paper Stomp, written by jazz trumpeter and bandleader Wingy Manone, who recorded the song in 1929 and 1930. After the success of In the Mood, Manone apparently was paid off by Miller and his record company not to contest the copyright.