Removing vocoder effect from audio file
I seriously doubt that there is a simple way to accomplish this.  Most of the work that I have seen tends to go in the direction of improving the synthesis, or using better (and more) phoneme recordings to get better quality.  If there were a simple way, I think someone would have found it by now rather than putting so much more effort into better synthesizers.

That said, you could try using a bandpass filter on the output.  The sample you referred to sounds better (at least is doesn't buzz) if you filter out everything above 800Hz, and everything below 100Hz.  That does kind of make it sound like the person speaking has a blanket stuffed in to her mouth, though.

If you would like, I could put together some python code (though you will need numpy/scipy for it) or you could use audacity or sox to see if the results are even acceptable before implementig code.