I'd just like to add my take on VO's for COT as well, since I am one of the lucky individuals who would be recording, editing, mixing and mastering any VO for MWM. Here are 4 quick reasons not to do VO in COT:
1. VO is expensive for quality work. It's safe to say a norm of $100-$250/page of script (~200-300 words), recording studio time (pretty much as stated before ~$250/hr) and other more nominal costs. It is true that my time is free and that helps a ton, but for quality i would still need to rent space in a studio for the quality that is expected. We could go cheap and do the VO with volunteers in a less than commercial studio, but that would just be...well...no. Better a game with great sound effects and music than a game with great sound and music that was interrupted by Uncle Pete's uncomfortably stilted cheesiness. BTW, another factor in the cost; we have TONS of NPC's in the gates already with more to come. They would all need voices. Which leads us to:
2. VO can actually be limiting. We intend to do continual issues on a regular basis and that means more dialogue, of course. No bid deal. However, what if something changes? The actor who did your signature hero's voice is not for hire anymore, the old dialogue ages badly compared to the new ones, new story arcs makes an NPC's dialogue from before take a different light that would have been misrepresented in some phrasing, etc. These could be handled, but it would take time away from other content, such as new powerset sounds, which are more important in the grand scheme of things.
3. Last, but not least, the voice just doesn't match up with your idea of what the character sounds like. We have all had that annoying experience. RP is an integral part of this community and such a juxtaposition would ruin things for a lot of people.
4. CoX did not have VO. Why? I am of the opinion that they had some of the same reasons as we do, but I also think it adds to the general appeal of the genre of superheroes. They are bigger than life, but silent in their most pure form: the comicbook. We are making a superhero game, but in essence we are making a comicbook game. Every voice that is "heard" is uniquely yours, just as your character is unique.
So TL;DR: VO is expensive, time consuming, somewhat limiting and rarely ends up being perfect. As much as we would love to be able to do it, we would like to avoid all of the negatives that come with it.