Electret microphone: how does it work?
It's a JFET and a JFET in this configuration self-biases its gate to produce some current between drain and source (although the simplistic circuit representation of an electret doesn't imply that).

Here's a graph that shows the JFET's drain-source characteristic. Note how the gate has to become negative with respect to the source in order to turn off the drain current: -