Is it possible to sense small audio signal like 30db with electret microphone? Is there any module to do this manually?
I guess the questioner sees that a low cost electret mic may well have high enough sensitivity (millivolts per Pa) so that low level sound, say 30dB, should generate a measurable output voltage.
He's right, but the self noise of the mic (it's said in datasheets as an equivalent sound level) may be as high or even higher than the voltage generated by the low level sound. In theory it doesn't be an obstacle, because stochastic process math gives how long one should average the result for a wanted accuracy, but measuring say 5 hours to have a certain confidence (for ex. 90%) that the actual sound level = 30...31 dB is not practical. In human perspective reasonably short averaging is possible if the self noise is substantially lower, say 20 dB or less for the said 30dB actual sound level.
The stochastic process math behind this all is unfortunately beyond my skill set, so I must skip it. If you are interested in it, you hopefully have a strong basement in calculus, probability and electricity. An university level communication engineer would handle the needed math.
Search for measurement microphones. You'll find some which advertise a high dynamic range, good even for jets and explosions, but watch the self noise or inherent noise, as some of the makers call it.
In theory one could make a cheap mic "better" by placing it into the focal point of a parabolic reflector. Or one could make an array of 10 or 100 identical mic capsules. These tricks really amplify the sound signal vs the mic noise (see NOTE1). They are useful for measuring or recording distant weak sounds, but they work only in free space where the sound comes from a certain direction. They are useless in measuring the chaotic sound field inside a room. There only a point sensor will make it.
NOTE1: 10 identical mics in series increase the signal output 20dB because the signal voltage is the same from every mic. The noise floor grows only 10dB, because the summed noise voltages are uncorrelated. A bunch of mics is bigger than one mic. If your mic array has diameter D you can still get good result also in a chaotic sound field if you filter out frequencies where the wavelength is 4D or shorter. Example with D=100 mm you should kill frequencies higher than 825Hz, which makes it useless for noise measurements.