I've looked it up and haven't really found a good answer (either nothing or I guess it stays "mixtape" with Spanish pronunciation?
I have to write journal entries in Spanish and I'm not sure about this word.
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I am not used to the term, but after a little research I think we just say 'mixtape' in English (well, with the English pronunciation we Spaniards have :-D ).
I'm pretty sure you can use the original English term without any problem, since the Spanish version of wikipedia and several Spanish journals and media do:
Be aware they use the femenine for mixtape in Spanish.
Disclaimer: I'm from Spain and have searched in Spanish media, maybe in South America they do have a specific term.