This is because you have your phone paired to your cars audio system using both the "Call Audio" (use for phone audio) & "Media Audio" (use for media audio) bluetooth profiles. The nav audio is considered media so when that is paired with your car it is sent to your head unit. If you were to switch your stereo to the bluetooth audio input you would hear the directions (this works much like if you have your phone connected to the aux jack in your car... even though you dont have the AUX input selected and are listening to the radio instead, the audio is sent out the phones headset jack regarless of the stereo's input).
This is good to know. Does this "break the pairing" between the car and the phone for media, or does it just temporarily disable it?
I had an issue with nav audio when i had my Droid Incredible 2 connected with my mazda's head unit... unfortunitly that phone would keep pairing the media portion of the profile everytime i started even though i had disabled it previously. I was happy to find that now that i have the GS3 i have been able to turn off the media profile (i currently dont use my phone very much to listen to music in the car - only for calls) and it doesnt reconnect evertime i start the car. Hopefully that will be the case for you too.
To remove the media profile
1. go to your bluetooth settings
2. Click on the gear icon next to your car stereo under Paired devices
3. Under profiles un-check Media Audio. (leave call audio checked)
This will leave the call audio paired with your car while the media audio (music / nav / etc) will use the phones speaker. If there is a time you want to use your phone to play music in your car over your stereo you will need to turn the media audio back on.
(note ... if you are also using a car dock that does carging (like the ibolt gs3 dock
iBolt.co - Stay connected with your world. ) you may have to go into the dock settings and uncheck the Audio Output Mode if you want the audio to come out of the GS3's built in speaker)