Audio detection works with Threshold Level (TL). This is not the same as sensitivity. When TL is set to a low value audio detection trigger and starts recording for a low sound. Using a low TL can also start recordings for noise, i.e. a sound that has nothing to do with the scenario. The TL should be set to a level that trigger the Audio detection and start a recording for a specific sound like the sound from the water and smoke detectors. If you set TL too low it will start a recording for noise. The level 0 to 255 is relative and depends on the output signal from the microphone.
Sensitivity is something else and works generally like if you have a high sensitivity it reacts to very low sounds. If one sets a sensitivity to a low value it must be very high sound levels to make the system to react. NCS use TL and therefore you had to go from 50 to 25 to have the Audio detection to start a recording.
The sound (level and frequency) from the detectors are adjusted to how our ears respond. The microphone can have a different frequency response and therefore the TL must be low for NCS to detect the signal. If you want to have a higher signal level from the microphone use another type of microphone or use an amplifier.
Using a frequency filter is certainly interesting and also very effective for noise reduction.