There is no doubt that you can legally download your own personal YouTube videos, public domain videos, and non-copyrighted videos. But you can't legally download copyrighted videos without the owner's permission.
According to YouTube Support:
“You can download videos that you’ve already uploaded. If you haven’t uploaded the video yourself, you can’t save it to a computer. You can download videos from the YouTube app to your mobile device only.”
A few things to keep in mind when downloading audio from YouTube videos:
- Downloading and converting videos without the permission of the content creator is against YouTube’s terms of service.
- Downloading videos may also be subject to local or national copyright laws.
- Whether it’s a film, a song, or a podcast, it is advised to check whether the copyright holder allows downloads from YouTube.
- It also makes sense to cite the original or source, if you are reusing this audio for commercial purposes.