How to get output voltage of microphone?
You assume the microphone is linear.

This means that when the sound level input to the microphone changes by 1dB, the output voltage of the microphone changes by 1dB.

Some old (very old) pointer type AC reading meters actually have a dB scale, but I've not seen any digital meters, at least down the hobby end, that offer a conversion from volts to dB.

To make a dB reading, you take a reference at one level, make your measurement at another level, then take the ratio. For a voltage ratio, the dB difference is 20*log10(ratio).

Conventionally, 0dBA is the threshold of hearing, not easy to judge.

So you have two issues. One, to get a meter that reads dB ratio, or use an Arduino or something that can compute and display dB ratio. Two, to establish your sound reference level.