This is the real purpose of C6 If t

This is the real purpose of C6
If the amplitude of the FM input signal suddenty increases the voltages VD1 and VD2 will try to inverease and these in turn will try to increase the voltages across both R1 and R2 . However since C6 is large the overall voltage across R1 and R2 will not respond to the fast change in input amplitude. The result is that the demodulated audio output is unaffected by fast changrs in the amplitude of the incoming FM signal.