- The arduino ADC is 10bit, which means the range is 0-1023 and this is mapped to 0-5V most of the time, unless you do funky things to AREF.
- Most amplifier circuits on the net (using op-amps or LM386s) will have a DC bias of 2.5V, this is to allow the output to swing both ways. So on the arduino side even with complete silence you will read a value ~512 or so.
- The resistor connecting the electret mic and power also adjust the sensitivity of the microphone in addition to providing power to the mic (which includes an internal pre-amp, by the way). This resistor is very important - if you find your microphone isn't responsive enough, then adjust this resistor first before debugging your amplifier circuit. For me this "fixed" my microphone's responsiveness. It is best of this was a variable resistor which makes adjustment trivial.
- I was making a simple volume meter, and I did the following: clamp upper values to about 600, then using map map this to a range between 0-7 (since I had 8 LEDs). This was because I found the value never went above 600 very often, so the upper few LEDs were essentially going to waste sitting around. YMMV.