How to download the highest quality .mp3 with youtube-dl on Sierra?

I'm trying to use youtube-dl to download the highest quality .mp3 files from youtube videos. I've installed youtube-dl and ffmpeg and downloaded libav. I found this command: youtube-dl -f bestaudio --audio-quality 0 --audio-format mp3 https://www.youtube.com/watch?v=3zy1SNH-VqE posted elsewhere, which is supposed to get an .mp3 of the highest quality, but it only downloads a .webm, and the filesize appears to be around the 128 kbps range. I used to get an error that said I needed to download ffprobe or avprobe, so I'm not sure I have the ffmpeg and libav installed properly. Also, I saw some discussion elsewhere, is ffprobe or avprobe better?