The 8555A has a "zero scan width" mode. In this mode, instead of scanning the spectrum it simply displays the amplitude of a fixed frequency as if it were an oscilloscope. Sounds suspiciously like an radio receiver, does it not?
Well it is. Just connect the auxiliary "Vertical Output" (bottom right of the pic) to an audio amp, tune to a station, and voila: audio. FM works just fine by tuning slightly off frequency via slope demodulation: remember, with FM the carrier is wobbling back and forth according to the audio, so if you go off frequency a bit and use something like 100kHz bandwidth, you can get quite nice audio because the carrier is effectively AM'ing as it goes through the receiver passband.
Here it is listening to 99.1MHz (CBC1) using a ducky antenna on the input (5 plugs up from the bottom right). Well, you can't hear it but actually it sounds really good.
The black box behind the loudspeaker is a 30A linear power supply. The audio amp is on top of it. That's an unused 144-148MHz Sinclabs cavity filter on the top right if you were wondering.
Still reading? You're an übernerd. Then you're sure to enjoy this page of analyzing a "brick wall" cavity filter.