Harmonizing is done by taking the signal while it is in the STFT form, the strongest frequency is found and multiples of this pitch and windowed (+/- n indexes around the peak) and shifted at multiples of the fundamental. In this implementation two harmonics are calculated. For a harmonizing effect of shifting and scaling the pitch by 4 steps and 7 steps (which will make a nice chord in many cases) the frequencies need to be shifted by a factor of 1.2599 and 1.4983 respectfully.

Signal Comparison (please click on the graphic)

M Files:
Example
harmonize.m

Wave File Examples:
Wave File 1 - Harmonize
Wave File 2 - Harmonize