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I don’t think that audio-only version comes from Youtube. I think youtube-dl downloads the whole video, then uses ffmpeg to extract and possibly re-encode its a

• yohui 12y ago
  No, YouTube does have audio-only streams. You can use "youtube-dl -F [url]" to view the available formats and their codes. They're meant to be paired with the video-only streams used for some resolutions (most notably 1080p). Even browser extensions such as YouTube Center can download the streams, and they certainly don't use ffmpeg. (In fact, I do use ffmpeg to combine audio-only and video-only streams via youtube-dl, e.g. "youtube-dl -f 137+141 [url]" which takes 1080p video and 256k aac audio.)