My only input is that a FM radio receiver does not produce any audio signal if there is no rf signal. They work by finding a rf signal, locking onto it then responding to the frequency deviations of the signal to create an audio signal. You'll have the baseline thermal noise from semiconductors but that's it.
On the other hand, an AM receiver will pickup and amplify anything including natural radio emissions along with a plethora of man-made signals and would be the best choice to use.
Indeed, totally agree there.
FM seams to be unsuitable for the purpose, I'm currently looking into MW/SW/LW, the more promising to be MW and SW, LW never been quite used so the band is pretty much left to static. The issue is; I don't know any module as the FM module referred to work with those frequencies, but I'm thinking on using a stepper to tune around an analogical MW/SW receiver circuit.