Unfortunately, Audrey Hepburn's efforts to improve her singing weren't good enough for the director. So, despite her months of training, while editing the film he decided, that she would have to be dubbed over. In the end, Marni Nixon sang almost all of Eliza's songs.
Nixon was an experienced dubber and had sung the songs for The King and I and West Side Story. Hepburn only sang the harsh-toned chorus for the song Just You Wait and its reprise. Jeremy Brett, who plays Freddy, was also dubbed by Bill Shirley.