Ok, it works for me.
Got in my car, set a destination in Google Nav. FM radio was playing in the car. I heard Google Nav tell me my first set of directions from the phone.
Then I saw the BT connect. I started to drive the route, and I heard nothing from the phone nor the car except the FM radio. Then, I pressed the "MEDIA" button, which normally scans for any music via USB or BT. It found my phone. My music player (PowerAMP) was already playing. Shortly after, I heard the Nav interrupt my phone's music with instructions.
So it seems like all audio from the phone speakers is redirected via A2DP to the car, but the car needs to be in the proper mode to hear it. Not sure if this is your issue or not.
If that doesn't work for you, try disconnecting BT and reconnecting from the car side.