Opera is the grand art that defined approaches of synchronization of music to image (and text and special effects and so much more). Richard Wagner’s massive operatic works are most closely associated with this approach. His friend Hans von Wolzogen coined the phrase “Leitmotiv” (German: leading motive) to describe Wagner’s approach.
Leitmotivs can be melodic, harmonic or rhythmic and are an emotional shorthand to the audience to signify or connect various scenes or characters in the picture.
Howard Shore’s score to “The Lord of the Rings” trilogy follows in this Wagnerian tradition and Shore composed 40-50 themes to be called upon again and again over the course of the 3 films.
Here’s three examples of how the Shire theme is used in different ways. Notice how it always evokes the homeyness and longing for the place they call home:
Frodo and Gandalf
Eric Rawlins has put together a comprehensive site on the Lord of the Ring’s Score.
John William’s has also effectively used this Wagnerian approach with the entire Star Wars franchise.
Contrast this approach with Tan Dun’s hauntingly beautiful score to “Crouching Tiger, Hidden Dragon.” Virtually every cue features the same main theme presented in different clothes depending on the scene. It’s incredibly effective and the listener/viewer never seems to tire of it. This can be quite effective in shorts or one hour documentaries and the like where there is just not enough time to support a larger structure of themes.