The schematic found here looks to me like it could be a good sound level meter with a few changes. One thing that it has going for it is the meter is inside of the feedback loop, so it will have a linear scale at low levels. Some meters don’t and the forward voltage of the rectifiers causes the scale to be distorted at low levels.
One problem it has is the coupling between the microphone and the opamp. The schematic shows a polarized electrolytic capacitor as the coupling capacitor. The problem is that every electret condenser microphone will have a different voltage drop across it when connected to this circuit. One ECM might have 4.5 volts or more across it, which would be ideal, since there is 4.5 volts on the negative side of the coupling cap and the cap would be correctly polarized. But another ECM might have less than 4.5 volts across it, which means that the cap’s positive terminal would be negative in relation to the other end, and the cap would be polarized incorrectly.
The easy solution is to replace the capacitor with a non-polarized capacitor such as a 1 uF plastic or disk capacitor. Then the polarity doesn’t make any difference. But there’s a much better way to do this.
Just remove the two 10 k resistors R1 and R2. Connect one end of a 100k resistor to the junction of R3 and C2. Connect the other end to the junction of R5, C1 and the ECM. When the circuit is first turned on, the R5 will supply a small amount of current to charge up C3 to the same voltage as that at the ECM. Since the opamp is a FET opamp, the inputs don’t require any current and the voltage across C1 will be zero. This saves one resistor, and the circuit can still use an inexpensive electrolytic for C1.
Back to experimenting…