How do audio signals recorded onto motion picture film as waveforms get converted into audio that can be heard, like in a movie, especially with different channels like stereo, and surround sound.
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Heyo, film projectionist here!
The waveform-looking sound on 35mm film previously used in theaters is read by a soundhead that projects a slit of light through the film onto a light detector (there’s many different kinds, but you can think of it as a little solar panel). At any moment in time, there’s an amount of light going through the soundtrack, which the light detector picks up, and with the film moving at a constant speed, a continuously varying signal over time can reproduce the waveform on the film. With some basic amplification, that’s enough for some very simple mono audio!
Modern film usually has 2 waveforms on the film, which can produce surround sound through some very clever encoding and decoding using an analog matrix calculation. You can read about that matrix magic Dolby figured out [here](https://en.wikipedia.org/wiki/Dolby_Stereo#The_Dolby_Stereo_Matrix). It’s simultaneously pretty simple and also quite complicated.
Dolby also came up with the black magic that is Dolby Digital sound on film, as well as the application of noise reduction to the standard analog waveforms on the film to make it sound better.
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