pito:
If you are interested in db, have a look at the AD8307 (Analog Devices) - it is a logarithmic amplifier which gives you 0.0-2.5V DC output (for your ADC) when the input signal is -80..+12dB (90dB range). It is a DIL8 chip, you need 1 resistor and ~3 capacitors and you get a calibrated db voltmeter.
Something like this is highly recommended. I've used that chip and it's very impressive in both it's dynamic range and frequency response. The arduino analog input is a simple linear scale of voltage. Sound level is generally stated in logarithmic scales as practical sound level measurements covers such a wide dynamic range. So unless your desired measurement range is very small, I think you will be very disappointed in the results unless you have an external logarithmic amplification stage.
Lefty