Actually, my understanding is that yes, every line is voiced by, err, hand

In other words, they've got voice acting for every line in the game. Even some random insignificant NPC abusing you for blocking their way home. All done by real live human voice actors, not by some in-game voice synthesis. 50 hours of voice samples, filling half the DVD.
The "whisper" idea is cute, Neela. It could only be used a few times before it became repetitive, though. You'll have to get your mod out quickly, to reserve the idea ;)
Momaw, Brood, I presume that having "unvoiced" dialog works, but the lip-synching is apparently driven by the audio, so that would mean the NPCs mouth won't move with a slient audio file. It will still be be odd when an otherwise-fully voiced NPC suddenly stops talking and starts just staring at you while text appears on the screen... telepathy, perhaps?

Oh well, we can always insert all-new NPCs into existing locations to link in mods. No problem with fully voicing a new NPC. It will just be a problem if it would make logical sense for someone already in the world should be saying something new (like as I said, the arch-mage, or some regular point of contact in one of the guilds, or similar).