We need mkvinfo and mkvextract from MKVToolNix and ffmpeg.
The former can be installed by running brew install mkvtoolnix.
The latter can be installed by running brew install ffmpeg
Sources:
http://macappstore.org/mkvtoolnix/
https://github.com/fluent-ffmpeg/node-fluent-ffmpeg/wiki/Installing-ffmpeg-on-Mac-OS-X
Usage: bash extract_audio.sh video-file.mkv