I'm inclined to say this was more developer laziness. Maybe the engine can't give creatures dialog? :unsure: Easy way around this is to make a suit of non-player-equippable armor that looks like a lich and have Manni wear that. Doesn't help with the wussy voice, though.
In oblivion, the only way to get creatures to talk was to actually not have the creature talk, but have the PC speak to an invisible actor who stood in roughly the same position and/or followed the creature.
For example:
The Damirask mod had a Xiviali (sp?) who could be talked to. The catch was that you were really talking to an invisible actor, and it was sometimes a real click fest to find the guy.
There was a mod (Mannimarco revisited) that did just what you're suggesting and fixed the wussy voice problem. (edited the sound file) You could also get the robes he was wearing and some other cool stuff.