Audio turn by turn directions are shut off when blue tooth is on in my 2011 Infinity M. The audio directions are shut off in the IPhone 5, and can't be heard on either the phone or the car.
That is an issue with the bluetooth device installed in your car. Rough guess, it doesn't acknowledge non-event based audio (IE incoming call). But I can confirm the feature does work. Use it in my car all the time.