ELI5, movies were made with photos that played very quickly, but how was sound added to that when it did?
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The soundtrack had, until films were replaced with digital files, its own place inside the film. Near the pictures there was a small line dedicated to the sound and to the music, in modern films it included even more than one track. Take a look [here](http://www.cineteatrolentate.it/files/audio.jpg).
Originally, they didn’t Early movies were just the video, with either live musical accompaniment (rare) or a soundtrack played from a record. This obviously had a lot of issues, live music was expensive and didn’t allow for any sound other than music, and sound on disc was difficult to synchronize with the film. Even a fraction of a second off was a huge problem with talkies, so a system needed to be devised that allowed perfect synchronization.
Which brings us to your question about how they went about getting the sound onto the film. There were several different processes, with a patent filed as early as the 1880s, but the technology didn’t really become viable until the 1920s. In short, the process involves encoding the sound from a microphone directly onto a special strip down the side of the film. A light on the projector would then read that strip as it played, decoding it and playing the sound in a process essentially similar to playing a cassette tape, but coded using light instead of magnets.
A video projector has a certain speed to show images at so that they appear to be in real time.
A record player has a certain speed to turn in order to sound like it’s in real time.
Make a machine for each where the speed is the same, or alternatively take two normal machines and connect them with gears to make the speeds match.
Then start the two at the same time, and they’ll keep in sync.
They can even both be on the same reel of film to make it easy.
The sound and picture used to be separate. At first it was a live group or pianist at the corner of the screen playing piano. Phonograph or record players could be used.
A magnetic strip on the film similar to tape was experimented with until the sound was encoded optically onto the film media.
[16mm film with optical audio track.](https://en.m.wikipedia.org/wiki/16_mm_film)
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