Short answers, pulled from the story.
The Lumière brothers screened their first film in Paris in 1895 while a pianist played live music to mask the mechanical noise of the projector. Early cinema proprietors turned to musicians because sound-absorbent walls did not exist between the projection machine and the audience.
Erik Satie composed what many consider the first frame-by-frame synchronous score for René Clair's Entr'acte in 1924. American composers Virgil Thomson and Aaron Copland cited Satie's work as a major influence on their own film scoring efforts.
Max Steiner and his contemporary Erich Korngold established the orchestral scoring tradition that defined the Golden Age. A landmark event in synchronization occurred with Max Steiner's score for David O. Selznick's King Kong in 1933.
Elia Kazan worked with Alex North on A Streetcar Named Desire in 1951. This score combined dissonance with elements of blues and jazz to create a modernist innovation using non-diegetic jazz.
In 1983, the Society for the Preservation of Film Music formed to preserve music manuscripts and other documents generated during score creation. These materials were often discarded by movie studios decades after production.