Having good sound is essential in having a professional looking movie. To do that, we need the proper software to get the cleanest sound possible. My favorite program to use is Audacity. Often I will bring my laptop onto a set and record the sound directly into Audacity so I can start editing the sound right away. This is a fantastic free software that has all kind of tools to clean up your audio and make it sound as good as possible.
Here are a couple more free options that I found: