The investigators sought the recordings between Manson family member Tex Watson and his attorney, hoping it could shed more light on the killings of pregnant actress Sharon Tate and four others at her Bel-Air home on Aug. 9, 1969, and grocery owners Leno and Rosemary LaBianca the next night, the Los Angeles Times reported Tuesday.
Watson was convicted in the LaBianca murders.
Challenges to Tuesday's ruling must be made in two weeks, court officials said. Watson's attorney had opposed the release of the interviews.
Police said it was a preliminary inquiry and that they don't know whether any new information would come from it.
Police sources told the Times the request is "routine due diligence" and not tied to any larger investigation involving the Manson family.
Watson, 66, the self-described deputy to Manson, was denied parole last year for the 14th time.
He was sentenced to death for his role in the murders but that sentence was overturned. While in prison, Watson married, fathered four children and divorced, began a prison ministry and became an author and ordained minister.
Manson was denied parole earlier this year.