This is not a trope, but rather trivia that is on the website of TV Tropes. The reason that it’s not a trope is because it is not a narrative device. It’s absence or presence has nothing to do with the story at hand.
Non-Singing Voice refers to the phenomena where an actor is cast in a musical film. However, the actor does not sing his or her character’s songs in the movie; somebody else does. The reasons are that the actor cannot sing or that he or she simply does not have the right voice for the material at hand.
This was most common during the Golden Age of Hollywood. Rather than cast actors who could sing, studios would cast whomever they felt was best for the role or whomever was a bankable actor. If the actor could not sing the songs in the film, then the studio would hire someone who could. This was done in secret because it was felt that it would ruin the film for viewers if they knew that actors were not really singing the musical numbers. Usually an attempt was made to make sure the singer could sound the way the actors might if the actors could actually sing; so in a way these singers were also acting the roles at hand. Such singers were known as playback singers and their actions for the films were known as dubbing; they would often sign contracts promising to keep their roles in the films secret. Eventually, the fact that the singers would dub famous actors became common knowledge, but there does not seem to be any backlash against the films themselves.
Now here are some noteworthy cases of the Non-Singing Voice.
Marni Nixon was one of the most common playback singers; she might as well be called the “Queen of the Playback Singers/Dubbers”. She dubbed for Deborah Kerr in The King and I, Natalie Wood in West Side Story, and Audrey Hepburn in My Fair Lady.
Natalie Wood appeared in several musical films and was dubbed in most of them. The most noteworthy is her role as Maria in West Side Story. She recorded her songs for the film, but her voice was considered sub-par because the songs were written for a higher vocal range than she was capable of. She was given the impression at least some of her voice would be used in the final film, but after shooting completed, she was informed that all of her singing would be thrown out, and that Marni Nixon would re-record her musical numbers. She felt betrayed. On YouTube there are clips comparing Wood’s voice to Nixon’s voice; it’s clear that Nixon has the more refined voice, and Wood simply lacked the range to adequately sing the songs. In 1962, a year later, Wood appeared in the adaptation of the musical Gypsy as the title character, and did sing her parts; it helped that the score of Gypsy was not as demanding as West Side Story. Wood would be dubbed two more times. In 1965 she appeared in the film The Great Race; Jackie Ward recorded the song “The Sweetheart Tree” which Wood’s character sings. Later that year Wood appeared in Inside Daisy Clover; she had three numbers, A slow and fast version of “You’re Gonna Hear from Me” and “The Circus Is a Wacky World.” Once again, Wood recorded all of those songs, but only her recording of the first four lines of the slow version of “You’re Gonna Hear from Me” appeared in the final film. The rest was dubbed by Jackie Ward. Ward later remarked that after she recorded the songs in the studio with the orchestra, the orchestra erupted into a standing ovation; when she asked why, they said that they were happy that there was finally someone who could sing the songs; Wood’s performances of the songs were apparently sub-par. Wood later stated that she was not against filming another musical, but only if she could guarantee that all of her songs would be recorded by her and appear in the final film.
In 1990’s several animated films featured voice actors who did not sing their musical numbers, and who instead had other people provide the singing voices of their characters. This included the Disney films, Aladdin, The Lion King, Pocahontas, The Hunchback of Notre Dame, Hercules, and Mulan. Other non-Disney animated films that did this included Anastasia, The Swan Princess, Quest for Camelot, and The Prince of Egypt.
Audrey Hepburn appeared in the 1964 adaptation of the hit Broadway musical My Fair Lady as Eliza Doolittle. Julie Andrews, who originated the role on stage, was considered, but was rejected as she had never shot a movie before. The role called for a soprano vocal range, that Hepburn simply did not have. She recorded all of her songs, but all of them were re-recorded, except for the majority of “Just You Wait” which was in a lower range than the rest, and one line of “I Could Have Danced All Night,” by Marni Nixon. Audrey Hepburn stormed out the studio when she learned that her singing would be replaced, but she shortly afterwards apologized her behaving so childishly. When it was somehow revealed that Hepburn was dubbed by Nixon, Nixon gained infamy, and it is rumored that this revelation is why Hepburn was not even nominated for her role in the film which won several Oscars, including Best Picture. Ironically, Julie Andrews made her film debut in Mary Poppins that same year and won the Oscar for Best Actress. There are clips available of YouTube comparing Hepburn’s voice to that of Nixon’s. Once again, Nixon is clearly more skilled for the material at hand.
In film version of Carmen Jones, much of the cast could sing, including Dorothy Dandridge, Harry Belafonte, and Diahann Carroll, but they could not sing opera. So, opera singers dubbed their voices. The exceptions included Pearl Bailey and Olga James; interestingly, Bailey’s voice did not sound operatic at all, and therefore, does not fit in the rest of the voices in the film (supposedly she refused to be dubbed) and James did have the ability to same opera. Similarly in the film version of Porgy and Bess most of the cast (some of them had appeared in Carmen Jones) was dubbed, but some were not including Pearl Bailey.
Interestingly, it seems rare nowadays for modern musical films to use playback singers. I think the reason might be because most actors nowadays have the clout to be able to sing their songs. Also and perhaps, audiences would not accept the fact that their favorite actors in the film are not really singing. Modern examples in musical live-action films inlucde George Clooney being dubbed in O Brother Where Art Thou by Dan Tyminski, Zac Efron being dubbed by Drew Seeley in the first High School Musical film (the others in the series had the songs written to fit within Efron’s vocal range) and Minnie Driver being dubbed by Margaret Preece in the film version of The Phantom of the Opera.