Sorry. MathWorks does not give out source code for compiled functions. So unless you are employed there, you don't get the source code in all cases.
HOWEVER, since all you need to do is to do this:
type voronoi
at the command line, you can see the source code. In this case, the code was written in MATLAB itself, and not compiled.
Note that voronoi also calls delaunay internally, and that code is compiled. So you don't get that code. You can check that, since type just tells you that delaunay is a compiled code. The same applies to any other supplied code from TMW. If it is written in MATLAB, then you already have the source code. If it is compiled, then you will not get it.
