Update: I’m sorry I don’t have the documentation, but a little while ago I heard someone on NPR say Marni Nixon’s work on classic films was uncredited because at that time people were under serious pressure to keep her work confidential. That conflicts with my earlier impression, and the content of this blog post was based on the early impression.
As the Milli Vanilli scandal proved, part of the time singers who dub voices go uncredited. We don’t have any way of knowing how often it happens.
However, there’s no real scandal when it’s common knowledge — or at least common sense — that someone in a leading film role lacks just one trait, and that one deficiency doesn’t warrant replacing the actor. The ideal acting, dancing and singing ability (not to mention screen presence) for a particular role rarely turn up in one person.
Still, for the sake of continuity, movie producers and directors prefer to have the audience listen to a beautiful singing voice while Deborah Kerr is on screen and not ponder whether it’s Deborah Kerr’s voice. This works best when the real vocalist is uncredited. As far as I know, movie studios, unlike at least one recording label, have never actively tried to mislead the public on that issue.
A link to NPR’s obituary for Marni Nixon appears below.