The interesting thing with this example in particular is that the audio output from the radio is already very well-gated thanks to an RF squelch (ie muted until a high enough RF level or pilot tone is detected - otherwise it would sound like an loud FM radio tuned to an empty band between transmissions and the gate wouldn't work). So internally there's already a voltage signal somewhere to indicate mute/unmute, with better precision than deriving it from the audio.

Of course it's a fun software exercise but if you were building a hardware product out of it you'd be better off splitting the audio into distinct transmissions based on the squelch signal rather than audio gate...

As it happens some modern avionic radio systems let you individually play back the most recent few messages for exactly the reasons the author describes (it's easy to miss)