A mixture of what Atomic and Kyler said. She's been in the business for a long time and learned lots of accents in that time. Plus, if she doesn't have a recognizable accent when one isn't required, e.g. in speaking to her colleagues or in a hostage situation, it's harder to make assumptions and facts about her so as to be able to pin her down (I realize that that doesn't apply so much in the former case, but the point still stands)>
I also realize that this got a bit long-winded, sorry about that.
If you're reading this you have 5 seconds to...Wait, Peaceful Apocalypse is a group of people now? Anyway, shot him/her a PM.
If you're reading this you have 5 seconds to...Hey guys! Sorry I've been MIA for a while; just got back from a trip. You've probably gathered this by now, but I'm trying to play Mouse as a genuine twelve year old, so she's going to be immature and resistant to criticism—at least at the beginning. I obviously don't think half the stuff she does is right, but it suits the character well.

True! Or she could have one of those robotic voices given to people who have a laryngectomy.
Don't look it up if you're a bit queezy. Not a pleasant sight. O_O