Before 1930's, before movies started having sound, they were completely mute. Music was already common in theatres.
Actors, in order to perform well and have the public's attention, would have to express themselves by face expressions and
The most common reasons for movies to be silent, was because cameras with sound were to expensive and/or didn't already exist, also, actor had not only to be good with face expressions and in order to have the public's attention, they would have also to sound good.
By 1930, with all the new talking pictures (movies with sound), music established itself as a vital element in the film industry.
In 1940, composers refined their expertise even more. One of the most important and influential composers was Bernard Herrmann, who broke many barriers and traditions to create music that greatly enhanced the films for which he wrote. With the development of Jazz, in the 1950s, Jazz made its way to be a great influence in the film industry. Even tho it was manly used for musicals and animated films, it made a because fewer musicians were needed, therefore less money would be spend.
However, during the 1970s, there was barely any change in the film music history, most people spent the decade perfecting things they had learnt from the last decade.
From 1980 until the 1990s, the first used of synthesised sounds in films happen in the 1980s, this lead to a major revolution. For the first time, it became theoretically possible to score an entire film with only one performer - using the synthesiser to produce the sounds of many instrumentalists.
Now a days, with the daily development of new technology and the general knowledge gained from the past, film composers have the ability to create perfect score, accenting the movies's plot and characters in such a way that it enhances the film and turns it into an experience.