The output level of an electret mic is about the same as from a dynamic or most other types of mic.
The circuit is found on a few sites in Google when looking for the Philips 3-wires electret mic. Its gain should be high enough for the circuit to feed a power amplifier or recorder that has a fairly high input impedance. What do you connect it to?
DO NOT bypass R3 with a capacitor because then the gain, noise and distortion will be much too high. Instead reduce the value of R3 until the gain is high enough and the noise and distortion are not too high.