My only input is that FM radio does not produce any sound if there is no signal. They work by finding a signal and then 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.