Continuity mistake: When Batman slams Joker's head against the glass after he says he took Rachel too, there are no pieces of glass on the wall shelf, but later a big piece of it alongside some more little ones is visible when Joker asks for his phone call. (01:30:00 - 01:33:05)
Continuity mistake: On lower 5th, the SWAT driver yells lock and load. When he does he has 2 cable ties in loops on his left hand side of his jacket that are exactly equal in size, both the loop and the "tail." We see outside the van for a few seconds. When the camera returns to the driver, the loops are now different sizes as evident by one having a longer "tail". (01:14:00)
Answer: It's habit, and a sensible one at that. When he's in the mask, when he's being Batman, he uses the voice, even if the person he's with knows who he is. If he drops back to his normal voice with some people, it sets the precedent for using it while wearing the cowl, which means that he's more likely to slip up and use it around people who don't know, potentially revealing his true identity. If he sticks rigidly to using the voice when kitted up as Batman, regardless of situation, it minimises that possibility.
Tailkinker ★