Personally I'm not sure, just spent 10 Mins looking.

Here is what Yahoo answers says.

You should not believe that music magically loses its copyright because it was "mixed" with something else, whether it's the lyrics or the instrumental. Because all music* published after 1989 is automatically copyrighted in the USA, you have to assume that any later compilations or "mixtapes" or other "collective works" would contain copyrighted works unless you can prove otherwise (e.g., music recorded prior to 1972).

Making mixtapes "copyright free" would be like saying your car is no longer yours because you parked it in a public parking lot with a bunch of others, even if all of the others had be legally abandoned.

*Of course not "all music" because works of US Government employees are not copyrightable, but that's not typically the most interesting music to have on a mixtape, unless maybe you're in the Services.
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But I think aslong as you aren't making money from them you should be fine.