The New York Times (nytimes.com) – ‘sound editing is about collecting the sounds needed for a film. Sound mixing refers to what is done after they are collected’

It is left to the sound editors to put in other noise to complement the spoken words. This could be ambient sound, like an air-conditioner or the approach of a distant train. In the case of “Star Wars: The Last Jedi,” this would mean the sound of Luke’s lightsaber in conjunction with the rustle of trees on Ahch-To, among many other sounds.

Sound mixing then refers to the postproduction after sounds are already inserted. The most essential goal is to meld the sounds seamlessly with the recorded dialogue. It could mean making that train actually seem distant or adding reverb to a bird’s squawk to create tension in a scene. It also includes matching the audio levels of dialogue recorded at different times.