Music and Sound in Film
The presence of a musical score in nearly all films is another factor that separates the study of film from that of most other texts. Analysis of musical scores has generally remained a more aesthetically oriented discipline than other aspects of film studies, for reasons one can only speculate upon. The largely non-verbal medium of the score might not attract the attention of scholars the way that the words and visual representations of a film do, for instance. In any case, this site will not attempt to introduce the principles of music criticism. We can, however, introduce a few terms that critics use to discuss music and sound in films.
Diegetic vs. Non-Diegetic Sound
The easiest way to divide all sound and music into film into two categories is to divide it into diegetic sound, which appears to emanate from the action portrayed on the screen, whereas non-diegetic sound (usually music) on a soundtrack would presumably not be heard by the characters in the fiction of the portrayed scene. When Ally McBeal sits in a club and listens to a blues band as she sulks about her miserable life, the band's music is diegetic. When Ally goes back to her apartment to continue sulking about her miserable life and we still hear the band playing, the music has become non-diegetic.
Classical Scoring Style and New Styles
By this point, you have probably picked up a pattern: traditional Hollywood film production strives to downplay the appearance of artifice in many ways, from directing, acting, and editing to--you guessed it--music. As traditionally conceived, music exists in a film to complement and enhance the emotional effects of the writing, acting, and direction. From this point of view, a successful score improves the experience of a film without calling attention to itself. Relatively recently, two alternative ways of thinking about scoring have gained momentum: one is postmodern scoring, in which the musical score attracts the viewer's attention by offering a commentary that offers meaning that stands in a contrasting or ironic relationship to the on-screen elements of the film. The other is pop scoring, in which films function in part as promotional vehicles for popular songs that will form their heavily marketed soundtracks.