Microphone Capacitor Value
R1 (RL in your circuit) determines the microphone gain and the output impedance of the microphone. Basically the output impedance is RL. If RL is (say) 2k2 then the amplifier that C connects ought to have an input impedance (resistance) => 22k. If it is 22k then C and the 22k form a high pass filter limiting the low frequencies and, at a frequency of \$\dfrac{1}{2\pi RC}\$, lower frequencies progressively attenuate more and more.

For 1uF and 22k the frequency is 7.23 Hz i.e. OK for audio - frequencies lower than this will get progressively more attenuated. A 100 nF capacitor would be fine when using the microphone for speech because the cut-off frequency would be 72 Hz.

I'd use a ceramic capacitor - probably X7R type.