How is sound recorded on motion picture film?

The phrase "optical soundtrack" sounds like a contradiction in terms, but it's the secret to how sound is recorded on motion picture film.

Everything You Need to Know About Recording Production Sound for Film

Summer 1998 - The Association of Motion Picture Sound

The First Sound Film Was NOT The Jazz Singer

Soundtrack - Wikipedia

16mm Sound-on-Film Motion Pictures - Lantern

Westworld monitors Westworld, Audio mixer, Music instruments

Movies & Film, HowStuffWorks

Experimentation with Sound

What is sound recording in film?