Download youtube videos as a mp3 using youtube-dl

so you only need youtube-dl and ffmpeg and if you want your mp3 file with metadata like the artist and title we can use the next command line:

youtube-dl --prefer-ffmpeg --extract-audio --audio-format mp3 --audio-quality 1 -o '%(title)s.%(ext)s' --metadata-from-title '%(artist)s - %(track)s' --add-metadata

Let's explain some of the argument

--extract-audio: Convert video files to audio-only files
--audio-format: Format of the audio file
--audio-quality: Quality of the audio can be between 0 (better) and 9 (worse)
-o or --output: How we want to name the file
--metadata-from-title: Parse additional metadata from the video title the format like --output
--add-metadata: Actually write the metadata in the file

Documentation