Movie sound is mostly recorded "double system" using a separate camera and sound recording, that way they can be edited separately for more flexibility and each system can use larger, higher quality, gear. One of the early exceptions was the Auricon camera which was aimed at news reel crews. The Auricon, later the Bach-Auricon had both a magnetic and a tiny optical sound recording system built in (the built in optical is the really hard to pull off part). When I was in film school a friend of mine bought a Bach-Auricon at a garage sale. Wishing to have it serviced in order to use it for making our student films he discovered that the Bach-Auricon company was still in existence in nearby North Hollywood. One afternoon he visited the factory, which looked completely abandoned but, after pounding on the door a frail old man finally appeared. The man was Walter Bach and He led my friend deep into what had once been a thriving camera factory but was now a dusty dark 1940s era industrial building. There was one light on over a work bench and Bach completely disassembled and serviced the camera while my friend stood waiting ... even occasionally helping out. He even repaired and adjusted the tiny and delicate moving mirror for the optical track. Walter Bach hadn't built a camera in many years and he no longer had employees but he kept on doing maintenance on the few cameras still in service. Eventually he stopped coming to work. The building was demolished in the early 2000s. A real Sunset Boulevard moment.