To get GPS voice over speakers - I have a 2012 Honda Odyssey, it must be attached to BT AND to the mode where you are streaming music (AUX button on mine). It won't go over the speakers if you are listening to the radio or XM. The unit will just be silent in that case. If you want it to just be over the S6 speakers then you have to disconnect the BT pairing for that to happen.
I am having an issue with the display on my Odyssey. With my old phone (HTC M8) when I was streaming via BT - the information would be shown on my display in the vehicle (artist, where it's streaming from, song, etc). It worked whether I was streaming from Pandora or similar and also if it was just via my music player on the phone. Now with the S6 it just says the source is from the S6 - but no song info will come over, it's just a generic screen. I tried re-pairing the phone but that didn't help. My guess is some incompatibility with Honda and the S6. It's a bit of a bummer. If anyone has any ideas I'd love to hear.