I have been struggling with this same thing, trying to hear turn-by-turn directions over my cars audio system through both Google Maps and Waze. I finally made some forward progress yesterday. I think the key is to find out where in your cars audio system you can set the 'source,' and set it to your phone. Full disclosure though, I only discovered this after poking around in the manual for my cars audio system.
Keep in mind that in Settings in both Google Maps and Waze, you can set it to where you want to direct the audio, either phone speakers or bluetooth. Oddly enough, I am only able to hear the turn-by-turn directions through Waze. I haven't gotten it to work over Google Maps, even though all settings are the same.