First, make sure you are not putting more than 3.3V into the analog input. Since you are putting 5V into the sensor, it is likely the peaks exceed the 3.3V limit. A voltage divider will fix this.
To convert the voltage into DB, you are going to have to calibrate the sensor. You will need a sound source of known output, and then develop a table of sound level to input voltage. You can then do some math magic to get a voltage to DB formula for the sensor. The formula will be logarithmic.
You might be able to find the conversion formulas on the datasheet for the sensor chip.