A sound editor is someone who decides what sounds should be in a film. They’ll look at a film without sound, and decide that they should have a sound effect of paws hitting the ground while a dog is walking. Or they may decide that there should be  sound for a ray gun.

The sound mixer is a bit different however. A sound mixer is given all of the sounds the edited has decided to put in the film, and the mixer will decide how the audience will hear the sound. That goes along with the volume, weather it increases or decreases, stuff like that.