According to imdb.com William Sadler is going to play a part in the movie “August Rush” directed by Kirsten Sheridan. The film is described as the story of gifted musical prodigies in love.
Freddie Highmore plays the main character August and William Sadler is going to play a person named Thomas. Jonathan Rhys Meyers, Keri Russell and Robin Williams are involved in this project, too.
See here the imdb.com movie page.