Impedance of a microphone?
I don't see anything to indicate that RL is \$2.2k\Omega\$. That resistor is there as part of the power supply to the microphone. If the resistor weren't there, then the output of the microphone would be shorted to the power supply, and then you'd have no output signal; the output signal would just be \$V_s\$.

RL together with the capacitor form a capacitive coupling. The signal we care about from the microphone is always changing, and mostly we care about the frequencies from 20 Hz to 20 kHz, because that's the range of human hearing. Higher frequencies can pass through capacitors, while lower frequencies can not. DC, being the lowest frequency (0 Hz) can't pass through a capacitor at all.

So, the capacitor blocks \$V_S\$ from the output. The signal from the microphone, which is AC, is less blocked by the capacitor than it is by the resistor, so most of the current goes to the output.