She was the invisible voice
Marni Nixon (February 22, 1930 – July 24, 2016).
Margaret Nixon McEathron, known professionally as Marni Nixon, was an American soprano and ghost singer for featured actresses in movie musicals. She is now recognized as the singing voice of leading actresses on the soundtracks of several musicals, including Deborah Kerr in The King and I, Natalie Wood in West Side Story, and Audrey Hepburn in My Fair Lady, although her roles were concealed from audiences when the films were released.
Nixon served as an instructor at the Donna Reed Festival for the Performing Arts in Denison, Iowa.