My suggestions:
1. Work on the voice acting and the upgraded S:AH should be separate as ShadowGorrath previously posted.
2. Mobius or ShadowGorrath post the script
3. Voice actors have to play the full campaign to get the overall feel as Mobius suggested. In the meantime, they will be also acting as testers and post bugs
4. As you might have noticed there are only a few who showed interest, compared to the total characters present.
a. Whilst practicing the script the voice actors can possibly come up with multiple characters
b. All voice actors post samples for in their opinion they are best in.
c. In the meantime, more voice actors can join in and we can invite more ourselves. I will ask IPAndrews of Bablyon5 for a list of voice actors he is using and maybe we can ask them if they are interested. Mobius, ShadowGorrath, are you ok with this?
5. The problem of who does what is not a problem at the moment. The answer will obviously be that the best guys for the part will be chosen, but it is useless to worry about this at this stage IMHO.
This is how I suggest we should go about it. However, feel free to scrap all this