Speaking of Stereo Audio vs Encoding Schemes, there was a great article posted here a while back about diving into audio formats on 35mm movie film [0]. There's a photo showing the analog stereo wave-form alongside two digital tracks: Dolby and Sony [1]. So the audio is physically printed onto the film in 3 different formats (1 analog, 2 digital) and it's up to the projector to decide which one is needs/has-the-hardware-for.

[0] https://news.ycombinator.com/item?id=30919904