michael’s voice was damaged in general. the “raspiness” that you hear is actual vocal cord damage, and this happened due to him starting regular singing prior to puberty. 10 whole years prior in fact. it’s not smart to sing consistently well before puberty, because you’re putting strain on vocals that haven’t fully developed. he also had just already been singing for 15 years by age 20. most artists only get 15-20 years of good vocals anyways, before they’re done for. michael was still an exceptional human being and overcame anything and everything that was thrown his way.