Sound editing is the recording and creation of sounds for a film piece, to create necessary auditory cues for different features of a scene, such as foley sounds for different props or creatures, real or fictional.
Sound mixing is the process of blending each piece of audio media together into one piece that suits the tone of the scene, and amplifies it. It involves lowering and raising the volume of each sound, and making use of both diegetic and non-diegetic sounds to further immerse an audience.