Offred's cassette tapes are mentioned in The Handmaid's Tale (Novel).
According to a symposium held in 2195, Offred’s story has been published as a manuscript titled The Handmaid's Tale. Her story was found recorded on a set of cassette tapes locked in an army foot locker in Bangor, Maine. An expert on Gilead named Professor Pieixoto talks about authenticating the cassette tapes pointing out tapes like these would be very difficult to fake.
The first section of each tape contains a few songs from the pre-Gileadean period, probably to camouflage the actual purpose of the tapes. The same voice speaks on all the tapes, and they are not numbered, nor are they arranged in any particular order, so the professors who transcribed the story had to guess at the intended chronology of the tapes.