What is legal and illegal to do in a rap mixtape?

Question

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Answer 1

As in you rap over famous instrumentals?

I think you'd need permission from the original artist or producer of the song.

Or at least give them credit somewhere.

Answer 2

Depends on what you do with it.  If you sell it, and the music isn't yours, it's illegal.  If you make it for yourself, it's legal.