Zero point (no audio) is also undefined.
Remove C3, and connect the output of the amplifier directly to A0.
Then you should have an A/D value of ~512 without sound.
You could use two variables. A 'minimum' and a 'maximum'.
Then sample for a certain amount of time, and update those two values on-the-go.
Subtract min/max, to find the peak/peak value.
Leo..