Wow! That's one seriously mis-conceived circuit!
OK, the discussion has progressed since I started writing - interspersing with other jobs.
As Mike says, "instructables" are dangerous - whilst some may be skilled, many are cobbling together basic circuits from venerable sources such as Don Lancaster's op-amp cookbook.
The split power supplies to the op-amp are clearly ridiculous. You just need it to be powered from whatever is your primary supply, whether 5V from USB or 9 or 12V as input to the Vin. Much the same applies to the electret supply. A single supply to a general-purpose op-amp such as a LM324 (somewhat overkill, as it contains four op-amps) with a "virtual ground" set to half the supply voltage can be AC-coupled.
And since you only want to detect level and not the signal itself, the output can be fed into a "charge pump" at the input of the Arduino.
