It isn't just an audio swap. There is an NFS file called Nicole that supplies the voice. Maybe it can be edited. I havnt poked at it much yet.
Maybe when the have the next update with text to speech we can edit a config file.
While poking around. I found icons for weather alerts of rain, thunderstorms and snow. Maybe Navigon is going to also have weather alerts for along your route? That would be pretty cool.
Most speed limits around here are 65 and I have mine set to warn at 20. I'd like to change it to say "If my calculations are correct, once you hit 88 you will see some serious ****."