I would love more romhacks that add custom voice acting. The issue is that finding anyone willing to work on this kind of project is difficult for two reasons:
1) Nobody who's serious about giving a good performance is going to want to work for free in this day and age,
2) Nobody who's serious about giving a good performance AND willing to work for free is going to want to star and be credited in a romhack with questionable legal ramifications, particularly if the game HAS been released outside Japan and DOES feature an English dub.
In my experience, there are numerous other problems as well. Hacking in custom sound effects is unbelievably difficult for most games, for example. Sound effects can have the right bitrate, length, filetype, header, and yet STILL not work. There's also the ethical question of whether replacing the voice acting is seen as disrespectful to the original English voice cast, as most redubs that happen in anime is done either for consistency, or they simply can't use the original voice cast anymore, either because they don't have the rights or they want to stick to non-union talent, not because they wanted to make a new one that was "better".
Is 2) a voice actor specific affair?
Most hackers and translators seem not to care, and legal cases are few and far between (mostly that PSP game that was the worst patch, as patching program, job ever and for a game set to be released at some point in the not too distant) or even utterly dismissed (Dead or Alive Volleyball way back in the day).
As far as ethical question... that is probably solved by *points at site we are on*. "if it bothers then you lean Japanese" tending to be the snarky expression of that one. We translate games, retranslate games, hack games six ways from Sunday on every front (levels, underlying logic, art...) and have done for years on this site, and going back further still... peek and poke on the commodore arguably counts and things predate that still.
There are no sacred cows (some in ROM hacking even revel in slaying sacred cows of the real world but that is a different discussion) and voice acting does not seem like a candidate for the first, either by logic (what makes a voice over any different to a script, visual art, music, game coding or any of the other) or pragmatism (there are various things we do around here as we kind of all have to get along).
I would also say voice synthesis provides a distinct possibility in working around finding candidates with the time, tools and talent (something that is no mean feat). If technically unsophisticated types can fake out family members and direct employees with minimal training data (to say nothing of millions routinely listening to text to speech channels on video sites that sound all the robot you ever want), adobe had PC grade software for it about 5 years ago, musicians have been brought back from the dead for decades now then I can't see this sort of thing being so very great a hurdle.