Tycho mentioned that I had the answer to the different clone voices in Republic Commando and that's true. I've seen a few people give the developers a hard time about this and it's not really fair. Please excuse me while I get really fucking geeky.
It's important to understand that commandos are not the same as clone troopers. They have been bred for an entirely different job. Clone troopers are "programmed" to function either as a part of an army or alone. Commandos are built to function as a part of a four man squad. They are separated from the regular clone troopers and they receive special training. Jango Fett himself actually assembled a team of the best warriors in the galaxy to train the commandos. Each teacher is the best of the best in his or her field and by coming to Kamino to train the commandos they have given up any chance of ever returning to their previous lives. They are for all intents and purposes dead to the rest of the galaxy.
So each commando would have spent countless years training with an expert from some far flung corner of the galaxy. It makes sense that they would pick up the speech patterns and character traits of their teachers. They are built to absorb information at an in incredible rate. So each of the commandos is going to interact with different teachers and have different experiences during their training and all of that is going to change him in different ways. So there you go.
If this kind of stuff seems cool to you then I highly recommend the Republic Commando book that's out there in stores. I've stated before that I enjoy terrible science fiction and in fact I read Star Wars books almost exclusively. So when I tell you that "Hard Contact" is a good read you can understand where I am coming from.
-Gabe out
