^That's what I'm thinking too, Petra. It's all terribly sad, though. I can't recall ever being that sedated. I mean, I've slurred my words a bit here and there, but I've never been so bad so as to sound borderline unintelligible like Michael sounds in this audio. I think it is to prove that 1)he knew what his "treatments" did to Michael, and 2) he continued administering them, thus, he is accountable and cannot blame Klein or any other doctor for his own actions.
As for the purpose of the recording existing at all, I think it is very likely that Murray was planning on selling/threatening to sell it to the media once he and Michael were through, which was bound to happen at one point or other. His decadent lifestyle and his various debts would steer him in the direction of extortion and blackmail... I certainly wouldn't put it past him.