I hacked together an Android app specifically for the purpose, so it doesn't really answer the original question about generally available projects. (I haven't got round to releasing the code as it's rather bespoke, but could do if you're interested.)
I FFT incoming sound buffers, and if the 2KHz band is the highest for a suitable number of consecutive frames, then I send the email and stop listening for a few minutes to avoid double notifications.
2KHz is the frequency of my machine's beep. Just triggering on overall noise level alone would notify me each time the machine goes into spin, but using the FFT allows it to be more discerning.
Perhaps someone (with more patience than I for Android/Kotlin development) can tweak it so that is can listen for knocking on a door to trigger some sort of alert (email/txt/broadcast an intent).
Triggering an intent would be very helpful as then it could then be acted upon by Tasker or Automagic4Android.
