I think, I found a solution.... It seems that different Bluetooth sets identify themselves differently - e.g. Headset or hands free... Etc... Therefore
If you have Cortana set to read messages over Bluetooth only, and your car hands free is recognized as headset - Cortana does not talk... To correct this issue make sure that "Read aloud incoming text messages" setting is set to "Bluetooth and headsets".
To change it:
1. Start Cortana app
2. Hit the horizontal bars icon - top right
3. Select "Speech Settings"
4. Make sure that "Read aloud incoming text messages" is set to "Bluetooth and headsets".
Once set Cortana should start talking to you through ur car stereo again
.... At least she does for me.....
Hope this helps.
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