Recently I have been doing some more experimentation with this. 

FM changes frequency moving from one to another based on the input audio signal.  This movement means that the amplitude at a frequency will change as the transmitter moves over it to follow the audio. This change in amplitude is what allows AM demodulation to hear a scratchy version of the FM signal. 

The next step is to try demodulation multiple areas of the FM signal with AM to get more of the total FM signal.