The bad news is we don't know the sensitivity of the mic..... That means you need an SPL meter to calibrate the SPL meter you are building!
dB = 20 x log(Vreading/Vreference). (We don't need the actual voltage since we know the ADC reading is proportional to voltage.)
Here's an example:
You play a test tone set to 80dB SPL on your SPL meter. You Arduino ADC gets a peak reading of 200.*
Change the volume of the tone and your ADC reads 400. Now we can calculate the relative dB change: 20 log(400/200) = 6dB. Add that to our 80dB reference and the sound level is 86dB SPL.
If you read 100 your dB calculation will be -6dB, which would correspond to 74dB SPL.
- Of course that's a totally made-up example. We have no idea what reading you are going to get at 80dB SPL.
And, since sound is pressure waves that go positive & negative and pass through zero, your readings will "look" random, but they will be constrained within a range and you can pick-out the peaks.
Also, the SparkFun board is biased at 2.5V, which means it should read about 512 with silence. You need to subtract the bias from your readings... A signal level that peaks at +200 and -200 will read between 312 and 712.
Note: It's not easy to make a "proper" SPL meter. Technically, you should be using RMS instead of peak level and you generally "weight" the readings due to the ear's different sensitivity at different frequencies. But, you can make something that works reasonably well for casual (non-scientific or non-regulatory) use.