Electronic – Why does the cable resistance jump from a low value to high value at a particular frequency

Your electret microphone probably uses a JFET transistor to provide hi-impedance to the condenser element whilst acting as a constant current circuit to the output. The constant current might be 0.3mA for instance and if you measure the current at two different voltages you'll get approximately the same current but, if you calculated the supposed resistance at these two different voltages you will get different values.

When powered by your circuit via the 10k resistor what you might find is that the (guessed-at) 0.5mA drawn by the microphone drops 3V across the 10k ohm and leaves about 5V across the microphone - this would appear to be like a 16.7kohm resistor where the microphone is. Obviously you got 17.5kohm and I've tailored the numbers to try and match this. Try measuring at the top of the pot and the 10k to see what voltage it actually is.

Now, when you measure with a meter you won't be supplying 5V - it'll be more like 1V (another guess) and the JFET may be less than perfectly biased and it will start to conduct more than 0.2mA - remember, when a JFET loses its bias it conducts heavily compared to the biased condition. So this might mean 1V and 1mA (not 0.2mA) which equals 1kohm.

S-parameters can be used at any range of frequencies that's the first point. The second point is understanding what a simple matrix of s parameters represents because two of the parameters are reflection coefficients and although they are of interest (generalism alert!) at any frequency, they tend to be ignored (because they don't offer any significant benefit) at (say) audio frequencies. The reason is because in audio, outputs tend to be low impedance whilst inputs tend to be high impedance. This kind of makes s-parameters to unwieldy for any circuit analysis other than when matched impedances are used. That leaves RF generally.