I just discovered that you can control Waze volume from inside of Waze on Android. First setup a navigation destination. Then tap the ETA bar at the bottom of the screen. When it opens tap the speaker in the upper right hand corner. Make sure that the "Sound On" icon is selected. Just above the "OK" bar select "More sound options". In the next screen that pops up you'll see a slider bar where you can turn up the volume specific to Waze. In my own phone the volume slider bar was set quite low. I'll try this out later today to see if Waze will now override Pandora.