The singer was found by relatives, unconscious next to a still-running treadmill, according to Steve Shular, a spokesperson for the Shelby County, TN Sheriff's Department.
Hayes was later pronounced dead at an area hospital.
The longtime singer and songwriter for Stax records released his first studio album in 1967, and his 1969 follow-up, Hot Buttered Soul, became a certified platinum hit.
Hayes' 1971 theme from Shaft earned him the number one spot on the Billboard Hot 100 and won an Academy Award for best original theme song, and was later followed up with Grammy awards for best original score and movie theme.
Hayes experienced a career revival in the late 1990's when he was brought on board the controversial Comedy Central animated series South Park as the deep baritone voice of the perpetually randy "Chef."
He left the show in 2006 after the creators lampooned his chosen religion of Scientology.
The Rock and Roll Hall of Fame inducted the singer into its ranks in 2002.
Officials do not suspect foul play in the death.
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