Rating loudness level with electret microphone
I designed this many years ago: -



It was part of an answer to this question. If you ignore the final transistor (Q3) and LED and take the output from across C2 you will have a more linear circuit for converting an AC signal into a DC level. To speed up the response you might choose to lower the value of C2.

Regarding the electret microphone, down forget it needs a supply bias usually fed through a 2k2 to 10k resistor from the positive supply rail.

It is capable of running from 5 volts (simulating will confirm) and will produce a DC level ranging from virtually 0 volts to about Vcc-1 volt.