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.

Motion Picture Sound - part 1

Summer 1998 - The Association of Motion Picture Sound

How Audience Testing Works

How Audience Testing Works

What Was The First Film With Sound? - Mack Sennett Studios

ISO 71:1977 - Cinematography — 16 mm negative photographic sound

Sound film - Wikipedia

Making Waves' Review: How Sound in Cinema Moves Us - The New York Times

The History of Sound at the Movies