The eight cassette tapes contain conversations between Charles "Tex" Watson and his attorney, the late Bill Boyd. Detectives want to listen to the tapes to learn whether Watson described any unsolved killings in the conversations.
Watson previously made the tapes available to the co-author of his 1978 book, "Will You Die for Me? The Man Who Killed for Charles Manson Tells His Own Story."
The ruling came despite an objection from Watson's current attorney, who argued Watson didn't waive attorney-client privilege when making the book deal.
Watson is serving a life sentence for his role in the Tate-La Bianca murders in 1969. Police want to know if Watson discussed other unsolved murders on the tapes.
The Associated Press contributed to this report.
