There are many questions about how to download video or audio from YouTube.
Here are the two tags [youtube-dl][1] and [youtube][2]. But in the
[YouTube Terms][3] 5.B., here I quote a portion of it:

> You shall not download any Content unless you see a "download" or similar
> link displayed by YouTube on the Service for that Content.

Are these questions allowed on Super User?

 [1]: https://superuser.com/questions/tagged/youtube-dl
 [2]: https://superuser.com/questions/tagged/youtube
 [3]: https://www.youtube.com/t/terms