The phrase "optical soundtrack" sounds like a contradiction in terms, but it's the secret to how sound is recorded on motion picture film.
AES E-Library » Motion Picture Sound in Record-Industry Perspective
Sound Editing vs. Sound Mixing: What's the Difference?
Soundtrack - Wikipedia
Who Invented the First Motion Picture Camera?
Sound Recording and Reproduction (Sound on Film) (1943)
Early Motion Picture Productions, History of Edison Motion Pictures, Articles and Essays, Inventing Entertainment: The Early Motion Pictures and Sound Recordings of the Edison Companies, Digital Collections
Motion Picture Sound - part 1
Kodak Basic Magnetic Sound Recording For Motion Pictures D.D.Teoli Jr. A.C. : D.D.Teoli Jr. A.C. : Free Download, Borrow, and Streaming : Internet Archive
Sound Recording and Reproduction (Sound on Film) (1943)
Sound film - Wikipedia