The phrase "optical soundtrack" sounds like a contradiction in terms, but it's the secret to how sound is recorded on motion picture film.
Motion-picture technology - Sound Recording, Sync, Mixing
The First Sound Film Was NOT The Jazz Singer
Sound Recording and Reproduction (Sound on Film) (1943)
How to Record Great Audio for your Films
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How is the audio recorded for a movie? - Quora
How Audience Testing Works
Movietone sound system - Wikipedia
AES E-Library » Motion Picture Sound Recording
Making Waves' Review: How Sound in Cinema Moves Us - The New York Times
Motion Picture Film Guidance: Identifying Motion Picture Film Formats