Please do not use youtube-dl to download copyrighted contents. Support the author/musician/artist.

There are some good mix on YouTube, SoundClound and MixCloud. Unfortunately it’s difficult to listen to them offline. When the artist doesn’t respond to my query, I use youtube-dl to download the mix.

youtube-dl --format bestaudio --extract-audio    \
--audio-format best --audio-quality 0 \
"URL"

Notes:

• youtube-dl already download the best available quality therefore no need to pass special options. However downloading the whole video to only use the audio track is a waste of energy, which is why --format bestaudio is set.
• --extract-audio still needs to be passed because the first option often download a video format with only an audio track which needs to be extracted/converted anyway.
• --audio-format is set to best by default and is most probably redundant
• --audio-quality 0 is the most confusing option: it sets the audio quality for the convertion (0 = better, which is itself confusing). However, with --audio-format set at best, one could assume a “lossless” (although files are already lossy anyway) extraction is done. In that case no need for a convertion, and, therefore, no need to use this option. But if a convertion is done, this option defaults to 5, resulting in a loss of quality. It is therefore set to 0 by precaution.
• --embed-thumbnail does not work with .opus (most YouTube videos)
• If using the snap version of youtube-dl, it seems to not be able to natively find AtomicParsley and therefore not beeing able to embed thumbnail

Knowing that, a simpler command can be used for audio only websites:

youtube-dl --format bestaudio               \
"URL"