That bothered me like crazy - how did they scan the audio whilst the frame was physically stopped? Then I realized how it was done: the soundtrack of frame A is stored in frame B, where frame B is far enough away that it has constant velocity.
There is a buffer!
Long:
So the frame being projected has to stop. Otherwise you'll get blurring unless your shutter is very fast, but then you'd burn out the film from the intensity of the light.
The film is stopped.
But, of course, you cant start-stop a 15 lb spinning reel 24 times a second millions -> itll all fall apart.
So the feed is a simple rotation at constant velocity (linear? Angular?). Then a piece of film ten to twenty (?) frames long gets put into a buffer (an empty box) and a mechanism akin to a sowing machine single steps through each individual frame.
Now the stepping mechanism only has to start-stop ten frames worth of film, a negligible amount of mass compared to the mechanism itself and the projector can last a long time of heavy use with simple maintenance.
Aint it neat?
The sync though is actually pretty easy, at least compared to computers and electronics - shafts and gears ensure synchronization with little effort (or you'd brake a shaft!)
Oh, I love ingenious solutions like that!