Note: general information, not legal advice. That said, some categories of content are clearly safer to convert than others. Here they are.
If you filmed it, recorded it, and own the rights to everything in it (including any background music), converting your own YouTube upload to MP3 is unproblematic. This is the clearest safe category. Watch out for third-party music in your own videos: if you used someone else's song, that part may still be theirs.
Creative Commons (CC) is a licensing system where creators pre-authorize reuse under stated conditions. Many CC licenses permit downloading and adaptation. Always check the specific license:
YouTube lets uploaders mark videos with a CC license; look for the license note in the video description.
Works whose copyright has expired, or which were never copyrighted, are free for anyone to use. This includes very old recordings and government works in some countries. Copyright duration varies by jurisdiction, so "old" is not a precise test on its own.
YouTube's own Audio Library offers tracks cleared for use in videos, and many independent artists release music under permissive licenses. Read each library's terms: "royalty-free" means you do not pay per use, but conditions (like attribution) may still apply.
If the creator says "free to download" in the description or provides a download link themselves, take them at their word, within whatever conditions they state.
Ask the creator, or stick to the categories above. See is YouTube to MP3 legal? for the broader picture, and YouTube's terms on downloading for the platform rules.
Yes, if you own the rights to everything in the video. This is the clearest safe category.
Content whose creators allow reuse under stated license conditions. Check the specific license for attribution or commercial-use rules.
Disclaimer: general information, not legal advice. Consult a qualified lawyer for your situation.