Sound Editing is CREATION OF SOUNDS, which can be based on real sounds [walking on 50 types of floors] or brand new sounds not found in real life [blaster sounds in Star Wars, dinosaur sounds in Jurrasic Park, Clickers in TLOU].
Sound Mixing, is taking all available sound effects, music and voice lines, and mixing them together to create a FINAL SOUNDSCAPE. Picking appropriate sound levels for each audio element, moving sounds in space where needed, making speech stand out by lowering levels of other sounds, very carefully managing sound in every cinematic [and picking appropriate sound behavior during gameplay], and more. This also include creating a stereo and surround mix [most easily noticeable with talking characters - when they are in camera shot their words will always come from the center speaker, when they are not in in camera shot, their words will come out from any other speaker [back pair for example]].
When Nolan fucked up Bane's speech levels in the first presentation of Dark Knight Rises to the press, that was a Sound Mixing fuckup.
