There is not much to be said. Circuit oscilates at 1/3 of the output
frequency. So what you hear is actually second harmonic.
I only tested this with standard 74 series, I can only guess that
74F13 could do better.
You can even connect a microphone to this little transmitter.
Just connect two wires to pins 5 and 7 of the integrated circuit, add
one 100nF capacitor in series and connect to the mic.
You can go as far as 50 meters or more.
So is this simple or what?!