Kurgan said:

The way I figured it all these years was that Boba Fett's voice was generated or distorted by his helmet. After all, that was the case with Darth Vader and the "Bousshe" disguise Leia used. It's true the Stormtrooper helmets only distort the voice without changing it (Luke & Han in ANH). Yet in the Prequels not only does Jango's voice just get the "stormtrooper filter" when his helmet is one, it's the same with the clone troopers. Jango's helmet is noticably different than everyone else's in the movies, why not just say when the helmet goes on he suddenly sounds like Classic Trilogy Boba Fett?

I believe that is Adywan's plan. Redubbing Jango helmet-on with someone who does a good Wingreen.