This 1969 Mullar circuit appears elsewhere on this site. The circuit uses germanium diodes and the indicator is microammeter with zero in the center of the scale.
This circuit makes use of a low impedance microphone. The relay is according to the supply voltage...
This circuit was found in an old Radio Electronics, but can still be assembled as the components are...
Any small sound amplifier from 200mW to 1W can be used in this simple circuit. Or circuit of two...