This circuit was found in a Popular Electronics from the 90's. The output signal can be applied to a headphone or speaker. The microphone can be a piezoelectric tweeter capsule or other type.
This circuit was found in Popular Electronics from February 1967. Transistors are from the period....
This page shows a crystal oscillator for 2 MHz. Other frequencies can be produced depending on the...
We found this circuit in an American 1980s documentation. The transistors can be equivalent as BC548...