This is kind of a wild guess since I haven't really studied fets, but it looks like it might be acting like a noise gate?
I'm basing that guess off of this article
If you leave the Gate (G) of a JFET open, the Drain (D) and Source (S) will pass signal through, as the switch is connected. If you pull the Gate to a voltage less than the Source, the JFET will no longer pass audio signal.
The drain's connected to signal input while the gate is connected to the output of the BJT. So, if the signal isn't loud enough, then no sound should pass?
I haven't built the circuit, so I don't know how it acts, but, like I said, that's my guess.