speakers - How to Turn the 555 Timer Output (Astable Mode) into a Decaying Square Wave?

I am currently working on a 555 timer in astable mode whose output will be fed into a 0.5W, 8Ohm speaker. I want a fading effect for the sound that will be produced by the speaker. I already acquired an envelope circuit which basically produces a voltage signal decaying exponentially for about 3-5 s. How can I turn the output of the 555 timer into a square wave that is also decaying exponentially within 3-5 s? AND can we also do that by only varying the voltage supplied to the 555 timer (i.e., the voltage connected to pin 8)? Here is the envelope circuit I already acquired:

Thanks in advance!