In Audio-Vision: Sound on Screen, Michel Chion explores the relationship between sound and image in film and the ramifications that this relationship may imply. Chion cites "added value" as the key to understanding the complex give and take between the audio and visual components of film. Added value is the concept posited by Chion that when combined, sound and image create a total effect that is greater than the sum of its parts. The effect of the two elements together is more powerful and expressive than either could be standing on its own.
Chion asserts that sound may have many faces as a counterpoint and companion to the image being projected. The most important aspect is language, as Chion posits that film is centered on the voice (the most significant aspect of which is communication). Moreover, sound lends context to the images presented, giving guidance to the viewer. Sound, for Chion, has the ability to fill in the gaps in the audiences perception of the images before him. Chion cites music as a second key point of sound. Music may be used two ways in film: empathetic (following the rhythms, tone, and emotion of the scene with which it is presented) or anempathetic (music that proceeds steadily, indifferent to the scene which it is supposedly representing).
What is most significant about sound in film for Chion is the effect it has on time and its perception. In a literal sense, the coming of sound cinema required a standardized timing for films (replacing the more haphazard projection of silent films). While necessary for sound synchronization, this shift also gave filmmakers power over the rhythm of final scenes as well as the total film, rather than leaving it to the mercy of individual exhibitors. In this vein, diagetic sound can act as a signpost to the audience, revealing the linear order of the scene (especially when using language). Furthermore, sound plays an important role in the temporality of the scenes themselves. Sound has the power to give a sense of linearity to what might otherwise be perceived as abstract images. Chion posits that sounds influences temporality in thee ways. The first method is "temporal animation," which refers to the ability of sound to change the viewers perception of the image from fluttering to static. The second way is "temporal linearization," when the sound appears to give comprehensive logic to what can otherwise appear as a discombobulated and disconcerting order of action. Thirdly, sound has the ability to "vectorize" scenes, driving them towards an ultimate goal or understandable ending. Different conditions apply to different filmic situations, and different methods may be applied to utilize the possibilities of sound and temporality. Chion gives four examples of how sound may impact a scenes timing. Temporality may be influenced by how the sound is sustain (whether its varied, whether its regularity, et al.), the predictability of the sound over the image, the tempo, as well as the sound definition (relating to sound timbre and frequency and how that may effect perception).
Key to understanding Chions argument, however, is his theory of reciprocity between sound and image. One cannot act upon the other without being changed itself. Each changes the perception of the other, but always in an equal proportion. Thus, sound reaches its full potential only through the lenses of added value.