ffmpeg is a free utility for processing video and audio files that can downloaded here. ffmpeg has many different functions. One of these is the ability to convert video files to audio files. If we go into Windows command prompt and enter this line
ffmpeg -i empire.mp4 -vn -ar 44100 -ac 1 -b:a 32k -f mp3 empire.mp3
A video file named 'empire.mp4' will be converted to an audio file named 'empire.mp3'.