Frequency de-modulation question
Your question is not at all clear.  What do you mean by &quot;message&quot;?
IF you are asking about the peak-to-peak or RMS amplitude of a demodulated FM signal - the absolute amplitude of the modulating signal is not a captured or encoded in an FM signal.  The recovered (demodulated) signal has the same relative amplitudes between loud and soft passages, but the absolute signal amplitude in volts is not modulated onto the carrier.