Hello and welcome to the Spotify Community!
I can't reproduce the problem, I'm using iPhone 5C with iOS 9.1.
When waze speaks the volume is low, after waze spoke, the music come back to normal volume.
Maybe I have to precise that the problem occures when Waze sound setting is on "Play sound to phone speaker".
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.
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