Once the recording process is done, it’s done. Noises, poor quality sound or the lack of it can spoil the best photography video ever or breaking the climax of the most outstanding performance. If a video sounds cheap it will look cheap regardless of how good the image is.
That’s when audio post production comes to stage, not only mixing the recorded sounds but fixing accordingly the problems happened in the live recording. That process includes meticulously balancing the different sound track levels, identify the unwanted noises, manipulating the original audio when it’s necessary and sometimes even adding “fake” sounds or studio voices. All for finally creating a tridimensional audio illusion to “deceive” the hears of the viewers and match an image, often has nothing to do with the really recorded sound source.