Piggybacking on OP's suggestion of youtube-dl, i have this alias for quickly grabbing the audio-only version of a youtube video. (I use it when I come across a song on youtube that I can't find for sale or streaming anywhere else)

alias dl-audio='youtube-dl --extract-audio --audio-format mp3'

If you have ffmpeg installed, just drop the video ID after dl-audio, and you'll get an mp3 of the highest quality audio stream available for the video.