It's true, Glenda's husband suspected them of having an affair so recorded them.
Her son, Damien, had actually sold them. Before the 2005 trial, Damien partook in a very pro-prosecution documentary called "Michael Jackson's Boys" where he talked about the family's time with Michael and played the tapes. Note he didn't say anything incriminating about Michael, in fact he just confirmed that Michael had more of an interest in his mother than he did with Damien or his siblings, but of course the documentary tried to portray it the other way.
There was a video on youtube with Damien's portion of the documentary but it seems to have been removed.
However, the Rhythm of the Tide website has a transcript:
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Damion: He adopted my family.
We opened up our family to him and he was kind of another member of the family. He got to experience what a family atmosphere was about. And I think that that isn’t something that he ever had himself.
Narrator: One of the most famous men in the world was now regularly driving out to Orange County to stay in Damion’s spare room.
Narrator: The Michael Jackson show got even more demanding, when the star began calling Damion’s mother day and night, to off load about his traumatic childhood.
Damion: He needed someone to confide in.
My dad would be waiting for my mom in bed, for her to to come to bed, and she would never come. She would be speaking to him in depth on the phone, late hours in the night.
Narrator: When Michael started calling three or four times a day, Damion’s father because suspicious and started to tape his telephone conversations.
It was extraordinary and flattering that a star like Michael would choose an ordinary family to confide his secrets to. But then Jackson suddenly stopped ringing.
Damion: And so we just waited on the phone call and it just never came. It was difficult to accept that.
Narrator: One minute Damion and his family were Michael’s best friends, the next they were sidelined.
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Here's a link to the website:
http://rhythmofthetide.com/tag/glenda-tapes-2/