Pre-recorded voices without street names, yes. But you have to replace each file with the same name and purpose. You cannot change the version with street names because those are downloaded as MP3 files on the fly from the server.
Icons can also be replaced.
You have to do all this with a file browser. And then, only on Android.
I was looking to make some car icons to kinda match the icons used for other users but from above and I'd love to use a GLaDOS voice like I use to have on my TomTom. Can anyone help me out? My searching has come up with nothing.