Using c++ and ffmpeg convert a video file to mp3.
Not using command line args, but using ffmpeg libs like libavutil, libacvodec etc
Basic idea is get a input video file identify the audio stream encode it to mp3 and write to a file.
If it is a MP4 input the audio stream will be in AAC , so transcode AAC to mp3 and write. Only this portion of code is required.