“What would you have done differently?” asked Bunny. “I didn't find Batman's voice all that bad [1].”
What would I have done?
Well, if you can suspend your disbelief enough to accept a billionaire playboy dressing up in a black BDSM (Bondage & Discipline / Domination & Submission / Sadism & Masochism) masked suit and beating up on criminals, then I suppose one could just have the actor portraying Bruce Wayne/The Batman use the same voice, same as every other incarnation of The Batman (although Bunny doesn't like that idea, claiming that people aren't dumb—“Hey! You sound just like that billionaire dude—Wayne something or other!”).
My other idea (and I suspect this isn't going to be popular at all) is to use two different actors—one for Bruce Wayne (I thought Michael Keaton [2] did an excellent Bruce Wayne in Tim Burton's [3] Batman [4], but made a lousy Batman) and a different actor for The Batman (I felt Val Kilmer's [5] Batman was much better than his Bruce Wayne in Joel Schumacher's [6] Batman Forever [7]). This would help (I think) to underscore the dualistic nature of Bruce Wayne/The Batman.
Either of those two options, in my opinion, is much better than having Christian Bale [8] imitate a 90 year old 5-pack-a-day smoker when talking as The Batman.
[2] http://www.imdb.com/name/nm0000474/
[3] http://www.imdb.com/name/nm0000318/
[4] http://www.imdb.com/title/tt0096895/
[5] http://www.imdb.com/name/nm0000174/
[6] http://www.imdb.com/name/nm0001708/
[7] http://www.imdb.com/title/tt0112462/