Also, do a search for "AM detector" and FM discriminator" to see how AM and FM are demodulated. Remember, AM and FM don't "happen" to the audio; The audio modulates (varies) either the amplitude or the frequency of a high frequency carrier signal. In the AM broadcast band, the carrier frequency is somewhere between 530 and 1700 kHz, depending on the station's assigned frequency. In the FM broadcast band, the carrier frequency is somewhere between 88 and 108 MHz, again depending on the assigned frequency. The maximum allowed deviation in the FM broadcast band is +/- 75kHz from the center frequency of the carrier.
FM signals are less susceptible to noise interference, because most sources of noise cause amplitude modulation, and FM discriminators are more or less immune to amplitude modulation.