I haven't actually tried this with Maps as I generally use only Waze while in the car. But, anyway, what I have found is that once I establish a Bluetooth connection with the car's audio system, Waze voice navigation always tries to push through the car audio, even if that is not the currently active audio output. In other words, if I am currently listening to FM radio or Sirius, the Waze audio simply goes nowhere. This happens in three different cars I have tried it in and the only solutions seem to be either to listen to my phone's music library on the car audio (and thus Waze navigation comes through over the music) or disable Bluetooth on my phone (thus Waze navigation comes through the phone's speaker instead of trying to route to the car's audio).
On the other hand, it's been a while since I have looked at this and I note that in Waze settings, there is now the option "Play sound to phone speaker" which was turned off. I will have to test this on the way home from work later today...
Interesting. Thank you very much for your response. I too will try enabling the "Play sound to phone speaker" option. That doesn't work with Google maps but maybe it does with Waze.