In fact, there is only a few minutes' worth of actual singing in The Jazz Singer. Jolson sings the song, "Mammy," twice during the film, with a couple of lines of dialogue. The rest of the film's soundtrack is instrumental musical accompaniment, and the standard caption cards prevalent in silent movies are used instead of recorded dialogue in most of the film. But that one song was enough to capture the imagination of moviegoing audiences of the day.
The movie was a sensational box-office hit, proving to Hollywood (and to the world) that "talking pictures" were profitable. It opened the door to the evolution of sound film, and it signaled the end of the era of the silent film.
This movie is one of those selected for preservation by the American National Film Registry as culturally significant.
See also blackface.