The clock is a 1950s Smith's Sectric synchronous clock, which runs on mains power. The way it works is using electromagnetism and there's a set of magnetic poles inside that oscillate at mains frequency.
The black box is one of those cheap phone amplifiers that work by picking up the magnetic field from the phone speaker and amplifying it. I thought they were a gimmick but no there's no microphone in one it is just a coil.
So I wondered, would that thing pick up the field from the clock and amplify it?
Sure does!
Random fun fact, it's a similar sort of principle to this (among other things) that Ben Burtt, sound designer for the original Star Wars movies used when he was creating the light saber sound. Only he used a microphone, and the back of an old TV set.