How to perform an action if the MIC has registered a certain sound

You mean like voice control in the phone?

In General, the only thing I found free on the subject — JackMeter (this is a command line program for linux audio JACK, JACK himself in difficulty settings is comparable to sendmail.cf).
For Windows, there are a whole lot of paid programs such as "Audio Spy", "Spy Voice Recorder", etc.
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Unfortunately, the only parameter to which you can attach the action — sound volume.

I suspect that to write such a program from scratch, not much difficulty will be, if you have experience in desktop programming for windows. I have, unfortunately, it is not.