How to put music that's legal in my videos for youtube?

You cannot legally duplicate or distribute music you got from iTunes. You are only licensed to listen to it yourself, and perhaps download to more than one device of your own. Royalty-free doesn't mean copyright free.

Owning a copy has little or nothing to do with ownership of the copyright. Technically, you or anyone else needs a license to give away free online copies to anyone who wants one. If you don't have that license, then you really don't have much of a leg to stand on.

However, if someone has unlawfully flagged your video (meaning they are NOT the copyright owners), you can certainly post a counter-notification with a copy of the "royalty-free" license you believe you have, and the copyright owners (if that's who they are) have 14 days in which to sue you and prevent YT from re-enabling your video. YT may, at its option, re-enable your video and remove the strike. You can, of course, flag the flagger for a violation of YT terms of service, if they illegally flagged you, not to mention suing them for damages, if you can prove any.

17 USC 512.

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