What I gather is that there are voice packs to replace the standard WoW voices that YOU hear, but other people would hear only the standard voices at their end. A very good thing, because I'd think that if 7 million players wanted their own voice files to be heard by other players, the servers would go nova with the strain :)
That, and I have zero interest in hearing what other people sound like. Anyway, some squeaky teen treble emanating from a Tauran? Not a pretty thought ;)