I just listened to it. It is electronic, not singing.
And it's not very convincing either.
Audio Restoration and Mastering Services
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> Date: Tue, 21 Apr 2015 06:24:45 -1000
> From: Malcolm Rockwell <[log in to unmask]>
> Subject: Re: Flexi Discs are back
> I remember this illusion from a presentation given around 1982 at the
> AES in Los Angeles at the Disney Convention Center. If there is an MP3
> available of that descending tone I'd like to hear it. I believe it was
> actually sung by a chorus.