We use the famous Open Source audio software Audacity to apply Noise Removal, Compress Dynamics, Equalization, Zero Cross, Silence and other filters. After the voice track is improved, we set a series of parameters for FFmpeg:
ffmpeg.exe -i path/video_filename -i path/audio_filename -map 0:0 -map 1:0 -vcodec copy -acodec libmp3lame -b:a 128k -ac 2 output_filename.mp4
It will show series of statistics from the processing being computed in regards to the frames, quality, time, size, etc.
Once it finishes the processing, the output file will be ready to be played by default on the bin directory of FFmpeg.