Using Google Maps and connected via Bluetooth, the navigation voice does not overtake radio audio unless Bluetooth Audio or Aux Channel is selected. This means that unless I'm listening to my own playlist (or dead air via Aux), I will not hear the navigation voice. Incidentally, the voice will come over the speakers if I'm on a phone call.
This happens with multiple makes/models of cars. In another forum, I read about incompatibility with G Maps & the default Music Player, so I uninstalled Music Player without any change.
Removing the phone from Bluetooth connectivity restores the navigation audio directly from the phone. I stopped at a Verizon Wireless retail store and one of the employees tried to solve this and could not. He uninstalled updates to G Maps I believe, to no avail, so then re-installed the update.
You're not the only one who has noticed this quirk.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.