[zirobi]

Note this will download the video and ffmpeg the audio out of it, so you're no longer saving bandwidth.

[DarylZero]

The -x option will do that ONLY if no pure audio stream is available -- for youtube.com, this seems only to happen for live streams. More common on other sites. (Youtube-dl supports many different sites.)

[rasz]

Incorrect. This will download

251 webm audio only │ 139.66MiB 134k https │ opus 134k 48000Hz medium, webm_dash

and then use ffmpeg to reencode opus to mp3