Unfortunately there is no "single output value". There is simply how accurate your microphone is picking up the frequency and how fast your arduino is sampling it.

I used that 2nd library you found (except I used the FFT version, which is pretty much the same)

The way it works is this. If you sample at 10,000 hz, then you can map frequencies 0-5,000 hz. Depending on your FFT_N size, you can get more and more accurate readings. Example: If you sample at 10Khz, you can map 0-5Khz, if your FFT_N size is 256, then the first 128 (maybe 127) bins contain useful information. This means that you have 128 bins to represent 5000hz. Thus you can calculte that each bin represents (5000 / 128) = 39hz.

So the problem with your 82.41hz is that it is so low that it is hard to pin down which bin it belongs to.

If you really needed to detect 1 specific frequency, then you may consider using an online calculator to calculate capacitor values for a low-pass and a high-pass filter. These filters will block out higher/lower frequencies.

Please read this thread for more information, I have some good code in 2 posts, so read all of it

http://forum.arduino.cc/index.php?topic=292834.msg2046136#msg2046136