If youre going to use FFT Results to check if a Note is singed by a Singer you should taken in mind that there might be Harmonics occuring like most Music Instruments have them, their Frequency is multiples of the Notes Frequency.
So if you Play an A4 (440 Hz) on the Guitar for example and check the FFT Results, you sometimes get 440 Hz as the Peak Frequency and also get 440*2 Hz or 440*3 Hz too as a Peak Frequency. So I would be intersted if that occurs on Notes which are singed by Humans too. Take a look at that Chart to see a typical FFT Result of a Guitar Tone: http://cnx.org/content/m13514/latest/guitar_A_220hz.png
So maybe could you post some Spectrums of Singer Voices or an MP3 to let me check it ? Since Im not a (trained) Singer I cannot check for myself if there are harmonics too.
The attached File shows a Spectrum of me trying to sing some tones, it looks like there are Harmonics too (Look at the thick Horizontal Lines).
Some months ago I was about to make a "Singin Trainer", an Application where you select for example 1 Tone and sing it for some Seconds and it gives you Points and a Diagram to check if the Frequency is correct, if the Frequency is stable etc and train it to get better. I was also thinking of some special Features like a "Stair" where you have to change from Low Tones to High Tones and the Software shows you every step you made up or down in Frequency.