Maybe the Bruce Wayne voice is fake.
In all seriousness, you'd be amazed at what a difference a new set of clothes does to your personality and how you present yourself. I wore a suit once, I felt so much more confident and outgoing.
He has to change his voice, so as not to be recognized as Bruce Wayne.
Superman/Clark Kent do the same.
His pants are tooo tight.
I KNOW RIGHT! I always thought he put something in his mouth