With FFmpeg, which you can download as a static build (no installation needed) for every major OS:
ffmpeg -i input.mp4 -c:a copy -vn -sn output.m4a
vn
and sn
disable video and subtitle streams, if there are any. -c:a copy
tells it to copy the audio bitstream as-is, without re-encoding the file. This will take less than a few seconds.