Yes, what GMAN said ...
Go to SYSTEM SETTINGS > English (US)
Same thing happened to me (July 2016, purchased in California).
When I picked the car up, the voice was set to be a charming English lady. I thought that would be kinda appropriate. But I soon realised voice nav guidance was not working ... and checkbox to turn it on was greyed out ... unable to be changed.
Turns out, the system needs to be set to US voice for US maps to have voice guidance.
I found this out in a roundabout way - I had removed the SD card (inside middle console) so I could plug it into my computer and download a free update I'd read about. I figured this update might get the voice guidance working.
While the card was in my computer, I was snooping through the files ... and found in the Nav file (or something similar) that only US English, US Spanish and French (presumably for Canadian buyers) languages had been included. Therefore, I realized my UK English selection in the car was not supported by files on the SD card. So, not working.
I hope this helps.