The noise originates from the 5V rail and may be due to: -
- Poor/unstable regulation from the 5V regulator
- Digital noise from other circuits not shown
The noise on the 5V line if, directly connected to the microphone's 2k2 will travel thru the 2k2 and superimpose itself on the microphone output and this will be coupled directly into the BJT amplifier via the 1uF cap. Hence the output of the BJT is noisy.
Without the 100R (i.e. shorted out) the noise will be somewhat attenuated by the 22uF and 100nF but, the noise may be quite low impedance and those caps may struggle to reduce it.
Placing the 100ohms in series with the 5V and hence the noise, weakens the potential strength of the noise and allows the 22uF and 0.1uF to do a much better job of reducing the noise. They won't affect the DC because they block it.
The 100ohm resistor will also slightly reduce the 5V but this is of no consequence to the electret microphone.
100ohm and 22uF start to reduce noise at a frequency dictated by 1/(2*Pi*R*C). This frequency is about 72Hz and any frequency above will be more seriously attenuated. For example at twice 72Hz (144Hz) the attenuation will be 6dB (half in terms of voltage). At 288Hz the attenuation will be another 6dB on top of that; a voltage reduction of 4:1.
It's a first order low pass filter and generally it's said that it attenuated at 6dB per octave or 20 dB per decade.
If the noise is low frequency AC hum or similar this filter won't do a lot for improving things but, if the noise is like white noise/hissy then it will significantly reduce the noise.