Downloading a YouTube video as an MP3 file with YouTube-dl, this command automatically gets the best available quality for the audio, saves it as an mp3 file with the video title and id in the file name.
All you need is YouTube-dl installed and the URL of the video you want as an MP3 file.
The YouTube-dl command is:
youtube-dl -x -f bestaudio --audio-quality 0 --audio-format mp3 https://www.youtube.com/watch?v=dQw4w9WgXcQ
-x gets audio only.
-f bestaudio the best audio format available for video.
--audio-quality is for FFmpeg audio quality 0 is best through to 9.
--audio-format mp3 output file will be .mp3
As no naming parameters have been stated the file will be named as videoTitle_videoId.mp3
For the example above:
Rick Astley - Never Gonna Give You Up (Video)-dQw4w9WgXcQ.mp3