iTunes U: How can I extract audio from video lectures?
I suggest using Max or ffmpegx, which is a GUI for ffmpeg.

You can also use ffmpeg from the shell:

for f in *.mp4; do ffmpeg -i "$f" -vn -c copy "${f%mp4}m4a"; done


-vn (video none) removes video streams and -c copy disables re-encoding the audio. You can install ffmpeg with brew install ffmpeg after installing Homebrew.