My first go at using MATLAB as a language was to build a phase vocoder with variable Q (below 1.5 works best) and create a GUI for easier use. It contains two methods of stretching Both versions of the source code are attached (GUI and non-GUI). Isn’t the fastest implementation so I would advise against trying to stretch large files. There are two algorithms included for stretching, the first use Overlap add of windows to create the stretch where as the other takes into account phase modifications

Non-GUI implementation Download

GUI Implementation Download