When do I need to obtain permission for the instrumentals I am using to make a mixtape?

21 Questions:
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- Is it when I am going to be selling the mixtape?
- What if I am giving away the mixtape, do I need a license?
- Say, I am an established, recording artist. And I make a mixtape using another artist's instrumentals, will I be expected to pay for the instrumentals even if I am giving it away?
- If I do need a license/permission for the instrumental for, say, 'Change Clothes' by Jay-Z, should I be talking to (a) Rocafella? (b) Star Trak? (c) Def Jam? How do I know who to talk to?

I mean, what are the rules? What and what do I need to keep in mind?

Thanks, thanks.