VHF spy circuits or Very High Frequencies circuits can transmit for a long distance. For that reason, you should check what laws you have in your country on FM transmitters. Here is the circuit, let’s see how it is built:
There are roughly less than 20 parts for this circuit. When you are building spy circuits, why not purchase a good supply of electric microphones. I would also use a breadboard to map out some of these circuits so that you could change them around and test them. See my article on using a breadboard for more details.