I always thought of Seppy as having a deeper voice than average, but still not nearly as deep as could be, and then very raspy, but still human-ish. Amadeus, on the other hand, is pretty much the deepest voice you could imagine... try imagining the sentence:
Amadeus: You solve everyone else's problems...
With some sarcasm. Make it very cold, and have his face and sarcastic impression in mind at the same time. That's my best guess at least.
Wargoth is like thundering flames, and MS is as dark and deep as you can imagine, like his voice was not coming out of him, but going into him from outside, like a black hole. NBoD and Xan are both pretty much like Tomix just described Xan. Warlic has a rather deep, soft, mysterious sounding voice. For Artix... I actually hear real life Artix's voice too often to imagine anything else than that. Cysero has a completely care-free voice, with a heavy american accent. Kara has a deep, mother-like voice, at least deep for a woman.
These are, of course, just the way I imagine them.