Building a spectral envelop of FFT'd audio
Two techniques commonly used for recovering the spectral envelope of an audio signal:


Low-order AR modeling. Estimate your AR coefficients, then plot the magnitude of the frequency response of the estimated all-pole filter.
Cepstral analysis. Take the logarithm of the magnitude of the FFT output. Take the FFT, truncate to keep only the first coefficients, take the inverse FFT, take the exponential. This is equivalent to applying a low-pass filter on the spectrum (with a log-scale magnitude).