Last year, I posted this question about a very simple RF transmitter circuit: Does this simple RF transmitter circuit actually work?
I built it myself, and actually remade it after I posted an answer to my own question; I applied CA glue to the coil so that it would stay around the same frequency. Though I think that power and even temperature(or air pressure?) could alter the frequency, it remains stable for a long period of time.
Anyhow, I am curious as to how I can add a microphone or direct AUX input so that I can actually transmit sound instead of silence.
I have achieved this to some degree by connecting an audio source to a secondary coil placed next to the main coil in the circuit. The second coil acts like an electromagnet and seems to modulate the frequency of the main coil. This does work, though the volume of the audio is low yet it is still prone to getting distorted at a certain volume. I'm guessing this has something to do with the frequency being only modulated in one direction(my understanding is that an FM "channel" is the center frequency for the modulation), and maybe the difference between high and low volume needs to be less significant. I'm unsure how to word it better than that.
I want to stick to the spirit of the circuit and go with something as dirt simple as possible. Indeed, the coil itself can act as a microphone but I've yet to make this practical or not prone to frequency drift.