He was administered CPR by paramedics, and taken to Memphis' Baptist East Hospital, where he was pronounced dead after an hour.
Now paperwork filed by Dr. David Kraus suggests that the star suffered a stroke, but a full autopsy has yet to be performed.
Hayes, along with Al Green, James Brown and Stevie Wonder, was one of the biggest black musicians of the early 1970s.
The musician even won Academy and Grammy awards for his relentless rendition of 'Theme From Shaft'.
It was in 1997 that he became the voice of Chef, the sensible school cook and devoted ladies man on the animated TV show 'South Park'.
The character won him great accolades, but he quit the show in 2006 in anger after an episode mocked his Scientology religion.