Despite frequently being ridiculed, according to the (now made unavailable) production notes for Batman Begins; Wayne is incapable of speaking without the Bat-voice when he is wearing his Cowl.
According to original drafts for the film:
"High-gain stereo microphones are concealed in the ears, allowing Batman to eavesdrop on distant conversations," the notes describe. The mikes can also amplify Batman's voice and broadcast it through a discreet speaker in the suit, giving his voice that distinctively gruff, disembodied sound.
While the original script included a scene that went over these
Bat-details, mentions of Batman's embedded vocal distorter wound up
being cut from the final film. (Still, the Batsuit's tiny speakers
remained visible upon close inspection.)
In discussing how he realized the mindset of Batman, Bale notes that his Batvoice contributed to him winning the role:
"I got there. They put me in Val Kilmer's suit. It didn't even fit properly, and I stood in it and I went 'I feel like an idiot.' What kind of guy walks around, dressed like a bat? And is then going to go 'Hello, how are you? Just ignore that I'm dressed as a bat.' Of course, he's meant to be doing this. If you look at the history of the guy and the pain that he went through. I went 'I can't do this in a normal voice. I have to become a beast in order to sell this to myself.'"
Whilst he isn't exactly 'going method' for the role of Batman, Bale indicates that for the character of Wayne, putting on the Batsuit is assuming a certain persona, as opposed to merely wearing a disguise.
In order to sustain the mindset of 'the beast', its plausible that when he dons the suit he becomes Batman, and this is a psychological state of which he is unwilling or unable to simply 'switch off', voice included.
Update: 'The microphone built into the suit' aspect of Batman's voice has since been validated by it's onscreen inclusion in Batman Vs Superman: Dawn of Justice