"It IS illegal to convert copyrighted music videos into downloads. That said, nobody has been sued for this (yet).
Again, it is completely legal to watch any video you want on YouTube. Streaming from a legitimate site is permitted under copyright law. And if it doesn’t involve creating a video mp3 or download, you’re in the clear.
But, it IS illegal to create a personal download conversion of a copyrighted work under US copyright law. That includes an mp3, mp4, or any other download file type from your videos convert process."
https://www.digitalmusicnews.com/2016/10/10/youtube-to-mp3-s...
It is certainly violating YouTube terms of service.
Given the video is available publicly, copying the video is what happens every time you watch it from your computer.
The conversion point doesn't apply as YouTube videos are mp4 and you can just play a mp4 as audio.
I'd argue it's covered by fair use but I'm not a lawyer.
What's copying technically and what's from a legal perspective is not necessarily the same thing (and one version can be allowed while another is not).
>I'd argue it's covered by fair use but I'm not a lawyer.
Fair use is a US thing.