What is a Phase Vocoder?
The phase vocoder is a variation on the short-time Fourier transform that uses phase information to improve the frequency estimates. It is ideal for use in applications such as time-stretching and/or time compression of audio, though there are a number of other special effects that can be implemented using the phase-vocoder strategy. This website describes the phase vocoder and presents an implementation in Matlab, along with a number of sound examples that demonstrate the operation. If you would like more detail (and better looking mathematical formulas) then you can download a chapter from Rhythm and Transforms which describes the PV and the STFT (among other things).
