I have the same problem on 5S with iOS 9.1. As soon as Waze speaks Spotify would stop playing. When I go to the Spotify app, the button is in the state of play (in other words, the pause symbol is showing as if it was playing). I Have to hit pause then play again to get it working. My Waze setting is 'play sound to phone speaker' is on and 'Prefer Bluetooth' is off.