Hi all,
just thought I might share the observation that voice output via bluetooth audio now works with WP 8.1 (Lumia 920).
I installed the developer preview of WP 8.1 and - voil? - I was quite surprised to notice that now the here drive + voice commands are properly played back via the car stereo which is connected via bluetooth audio.
The radio or USB playback is muted for as long as the voice commands are running.
With WP8 it only worked while streaming media playback from the phone via bluetooth a2dp (e..g streaming of music, not the phone audio).
So apparently it was an OS limitation that prevented this to work with WP8!