Find similarity of sound signal
To compare the similarity between multiple pronunciations of a single word, the spectral power that you've computed is too general a feature and doesn't capture details of the shape of the vocal tract, where variations in pronunciation may come from.

I recommend you compute Mel Frequency Cepstral Coefficents (MFCCs), which is widely used in speech recognition systems because it captures the envelope of the short time power spectrum of the vocal tract. You can build on your FFT code, but there are also many good toolkits available. 

Once you have MFCCs sequences for each pronunciation you can use Dynamic Time Warping to compute the corresponding "distance" or "similarity" between them.