So, the smartdrive 66 or any Garmin device designed for automobile use only receives audio from the connected device to be played through the Garmin. So, the GPS lacks the capability to send audio to another device that can receive audio, such as a headset.

Is this a hardware limitation or something that could potentially be programmed?

It's not a major issue to not be able to hear upcoming turns, but it would be nice. So I thought I would search the internet for more information.