a microphone would give you an AC signal ("frequency wave" as you call it), not a DC voltage. To get a DC voltage, you can use a simple 3-stage analog circuit: amplifier, peak detector, low-pass filter.
For the amplifier you can use a super simple, standard op-amp circuit (one chip and a couple resistors), for the peak detector: one diode and a resistor and capacitor, and for the low-pass: a resistor and capacitor. In the end, you're looking at maybe a dozen parts, though you might need a couple of amplifier stages depending on how weak the output of the microphone is (I've never worked with one myself)
I built a circuit just like this a couple weeks ago, only I was using line-level audio as my input so it didn't require much by way of amplification.