It was created as part of a discussion in the comments here:
That discussion can be referred to, for the detail!
Example output when placed several meters away from a battery-powered smoke alarm which has just started to sound:
Here is the general circuit. It accepts an electret mic element input along with a 3.3V supply, and outputs on J1 pin 2 a signal suitable for listening via an audio amplifier, or alternatively J1 pin 3 can be used to directly connect to an analog-to-digital converter (ADC) input on a microcontroller.
For alarm sounds, if it is desired to reduce the frequency response for normal speech, then C1 and C3 can be reduced in value, for instance C1 can be 100nF and C3 can be 10uF.
The printed circuit board (PCB) layouts are below. The attached zip file below can be sent to a PCB manufacturer. The board size is 37 x 15 mm.
All the parts are large and easy to hand-solder.