Ffmpeg
Convert Video Format
Set bitrate same as audio bitrate from original file
ffmpeg -i input.m4v -ab 192000 output.mp4
Convert Video to Audio-only
I don't think -f is required, as ffmpeg will otherwise use the file extension to determine format. -ab specifies audio bitrate. -vn specifies no video.
ffmpeg -i input.mp4 -f mp3 -ab 192000 -vn output.mp3