August 6, 1926 marked the premiere of Don Juan, a film about a charming womanizer. It also marked the first time audiences were presented with a feature-length film complete with synchronized Vitaphone sound effects and a musical soundtrack.
At this time a movie soundtrack “was not printed on the actual film, but was issued separately on 16-inch phonograph records.” These records would then be played in sync with the movies themselves to provide audiences with some lovely sound. A setup like that is referred to as sound-on-disc.
Vitaphone certainly wasn’t the first such process, but it was “the last [and the] most successful” because of its fidelity and amplification. It is said that it was “critical in stimulating the development of the modern sound reinforcement system.” [Wikipedia]
