I pretty sure turn by turn via Bluetooth is not supported as of this time on imaps. It works fine on my Navigon, and Waze app. Checking the new maps description on apples website it doesn't mention turn by turn using Bluetooth.
I just tried this with my 2012 Lincoln MKZ that also has the Sync system.
At first, the system behaved as you described in your original post. That is, there was no sound from the phone or the car speaking the turn by turn directions. However, I then changed the input source on the car's audio system to Bluetooth (the default was "Line In"). I'm not sure if your system is similar, but what I did was push the Media button and select "BTST" (Bluetooth Streaming) on the display panel. After making the settings change so that the input source was Bluetooth, I could hear the iPhone voice and turn by turn instructions from the phone on my car speakers. I also noticed that in addition to the turn by turn directions, I could also hear any music that was also playing by Bluetooth from the phone.
Hope this helps.