Reaction video should not so big of an obstacle because you can hide behind the fair use regulation but mixtapes you need to get permission. Best is to contact the artist or record label if you're not dealing with an indie artist.
I own a radio station, and all music producers send me music are with CTA (copyright transfer agreement). This way my station don't have to pay a thing and is protected to all claims (in and outside YouTube) because the copyright is transferred to my station.