Sound Editor:

Basically, the sound editor is responsible for all of a film’s sound elements, including dialogue, sound effects, automated dialogue replacement and more. It is not until all of these elements are brought together that the sound mixer can get to work.

Sound Mixer:

Once all the different sounds are edited together, the sound mixer comes into play. According to Variety, the sound mixer ultimately determines how an audience hears everything in a film. The sound mixer must decide what elements to emphasize and which to tone down. For example, there may be points in a film where the sound mixer feels the score should reign supreme. At other points, the sound mixer may emphasize the heavy breathing of a protagonist, perhaps during times of extreme stress. 

Info sourced from: https://www.businessinsider.com/oscars-what-is-the-difference-between-sound-mixing-and-sound-editing-2017-2?r=US&IR=T