Police officers routinely record interactions with citizens when they suspect there’s the possibility of a beef. That’s one possible source of tape. The other is from the radio traffic. Anybody yelling in the background would be picked up during transmissions from the scene. I’m awaiting release of the tapes with great anticipation.
My understanding is that it was a microphone on his radio or something like that. In any case, the unilatercation was heard in real time by folks at headquarters and recorded there.