audio only converter and expand the cropped video

Some may only want to change the audio to mp3 or supported format, without encoding the video , which take hours to finish. a feature that allows user to just copy the video as original format but changing the only audio part, which should be finished in a few minutes. iF FOR example, an mp4 file with mp4a may not play in video, but mp4 with mp3 in h264 video format would play.
Second suggestion is while crop a video, i want to fill that with black colour so that i could again include subtitles