Why An FM receiver recovers audio by measuring instantaneous frequency deviation against the center of its discriminator (detector) response curve. That curve is only linear over a limited range centered on the carrier, so if the incoming signal sits off to one side, the deviation swings run into the curved, nonlinear part of the response and the recovered audio comes out distorted, often sounding harsh or clipped on loud peaks.
Watch out Rising or falling pitch is what you hear when you mistune an SSB or CW signal, where the tuning offset shifts the beat frequency directly; FM detection does not work that way, and a simple amplitude drop is what you would expect from a weak signal rather than a mistuned one.
Mistune SSB and the pitch changes; mistune FM and the audio just gets ugly.