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.

(I'll try to publish the code soon.)

I'd appreciate seeing your code for this - I've intended to write a similar app for listening to an old alarm system I have.

I've now put it on GitHub: https://github.com/martin-stone/AndroidAudioNotifier

Thank you!