I just went from a Oneplus One, which did audio ducking properly - however the Nexus 6P doesn't, it fully interrupts the sound every time, even clicking a 'Like' on FB. I have no settings I've been able to find, and I want the ducking on notifications (and nothing on clicking a 'like', that's just nonsensical)
The tray is also inconsistent - very often Spotify simply disappears from the notification bar for no apparent reason. Opening the app makes it reappear.
This could also be an incompatibility as the 6P presently runs Marshmallow.