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A soundtrack album is any album that incorporates music directly recorded from the soundtrack of a particular feature film or television show. The first such album to be commercially released was Walt Disney's Snow White and the Seven Dwarfs, the soundtrack to the film of the same name, in 1938. The first soundtrack album of a film's orchestral score was that for Alexander Korda's 1942 film Jungle Book.  However, this album added the voice of Sabu, the film's star, narrating the story in character as Mowgli. 
Sometimes all of the tracks from the movie are included in the album, especially on a CD release of the soundtrack as opposed to an LP one, but more often this is not the case. There are sometimes instances of songs in the trailers that do not appear in the movie but occur on the soundtrack album. In addition, music that is not in the movie can sometimes be incorporated onto a soundtrack album; one notable example is the South Park: Bigger, Longer & Uncut soundtrack. Two other well-known examples are Rodgers and Hammerstein's Carousel and The King and I. In each case, two or more songs which are featured on the album were not included in the finished film.
Often, in advertisements or store listings, soundtrack albums are confused with original cast albums. These are albums made with the original stage cast of a Broadway musical, and are recorded by the cast either in live performance or in a studio, not transferred from a movie soundtrack.
In some cases, recorded dialogue may be incorporated into the soundtrack album. This comes in two kinds: audio clips from the movie itself (used on the albums for Pulp Fiction and Apollo 13, for example) or radio dramas that involve the characters from the movie involved in other events (example: King of Pirates, from FLCL). The unusual first soundtrack album of the 1939 film The Wizard of Oz, issued in 1956 in conjunction with the film's first telecast, was virtually a condensed version of the film, with enough dialogue on the album for the listener to be able to easily follow the plot, as was the first soundtrack album of the 1968 Romeo and Juliet, and the soundtrack albums of The Taming of the Shrew (1967 version), Cromwell, and Little Big Man. In the case of Patton, the bulk of the album featured the film's musical score, while the opening and final tracks featured George C. Scott's opening and closing speeches from the movie. The highly unusual soundtrack album of the 1972 mystery film Sleuth was designed as a sort of teaser, with Laurence Olivier and Michael Caine's voices heard for the first three minutes, after which the dialogue was abruptly cut off and the musical score of the film took over, forcing listeners to "actually see the film if they wished to know what the mystery was all about."
In a few rare instances, the complete soundtrack for a film — dialogue, music, sound effects, etc. — has been released. One notable example was a 3-LP set of the 1977 Rankin-Bass film The Hobbit. Because this particular film was produced for television, it lent itself well to the LP format: built-in commercial insert points were used to end each LP side, thus avoiding any additional editing. Another example was the above-mentioned Zeffirelli Romeo and Juliet - the movie proved so popular that two years after the film's original release, an album set of the complete soundtrack was released. Still another example was the Laurence Olivier Richard III, the soundtrack of which was released as a 3-LP album by RCA Victor in 1955. 
Soundtrack albums account for the bulk of the Indian music industry. In India, filmi soundtrack albums from Bollywood (see Music of Bollywood) and other Indian film industries usually sell more than Indian pop records.