Sound editor-

an audio engineer who oversees the production audio crew, mixes and balances the audio as it is recorded, and works to identify and solve the many problems that arise in this field: background noise, echo, distortion. (berklee.edu)

Sound editor oversees what happens to sound during production.

Sound mixer-

creative professional responsible for selecting and assembling sound recordings in preparation for the final sound mixing or mastering of a television program, motion picture, video game, or any production involving recorded or synthetic sound. (wikipedia)

Sound mixers edit the sound into the film. post-production