Convert Video Audio to MP3 with FFmpeg

To rip the audio from a video file and encode it to mp3:

ffmpeg -i MOVIE.webm -q:a 0 AUDIO.mp3

The -q: sets fixed quality scale (VBR) to the selected stream, which is a (all audio streams), and passes 0 to the audio codec (LAME) which interprets it as "highest quality". 9 would be "lowest quality".