When using Google maps in my car. The voice for directions etc works sometimes then not others. I'm not adjusting anything which makes it even more idd
I had similar problems in the past. Realised my phone had connected to the Bluetooth on the car but because Bluetooth wasn't selected as source I couldn't here anything.
You need to select Bluetooth on the radio to get the sound. Alternatively you should be able to set the audio output on your phone to play through the phone speaker.