Trivia: In the 40th anniversary re-mastered edition they have used Audrey Hepburn's original singing track for the "Wouldn't it be lovely" song.
Trivia: Audrey Hepburn was not the only voice that was dubbed. The actor who plays Freddy had his voice dubbed as well, so in their scene together with the song 'show me' neither of them are singing.
Trivia: Cary Grant was offered the role of Henry Higgens by Warner Brothers. He turned it down and said that if any one other then Rex Harrison, the actor who was Higgens on the stage, got the part he would never do another film for WB.
Trivia: Rex Harrison was scared to death of going into a sound studio to record his vocals. He was allowed to sing "live" directly onto the soundtrack, as the film was made. Incidently, Alan Jay Lerner wrote the lyrics specifically to be half sung/half spoken. They were written especially for Harrison, as he was the one and only choice for Higgins when they originally wrote the musical.