How to hook Raspberry Pi with Electret Microphone to detect sound levels in dB?
You selected a microphone board with automatic-gain. This is exactly the OPPOSITE of what you need.  You can not measure the actual audio levels when there is something upstream UN-doing all the audio level changes ("auto-gain").  So first, you must use the proper microphone module that will deliver the actual, honest, un-modified audio levels to you.

Second, you are trying to measure audio LEVELS, not the actual audio WAVEFORM itself. That means you need to average or integrate the audio signal to produce the "envelope" of the audio levels. You can do this in software by sampling the audio at some high rate and furiously do mathematical calculations, or you can do this much more simply in hardware by rectifying and integrating the audio signal into a moving DC signal which represents the audio LEVEL at any particular moment. Then you can sample and scale this varying DC into deciBels or whatever.

I would very strongly recommend getting a microphone board essentially designed to do exactly what you are proposing.  Namely the Sparkfun Sound Detector board. This board has the electret microphone capsule, and also the mic preamp, and a peak detector and an output buffer amplifier.  All you need to do is connect the peak detector output from this board to an analog input pin on your Arduino and you have the audio envelope delivered to you on a silver platter.  Easy-peasy.

Example: https://www.sparkfun.com/products/12642