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

 ffmpeg -i input.mp4 -f mp3 -ab 192000 -vn output.mp3

-f

output file extension

(I don't think is required, as ffmpeg will otherwise use the file extension to determine format.)

-ab

specifies audio bitrate

-vn

specifies no video.