I have just observed some strange behaviour involving VoiceOver, Audio Ducking, and Airplay which I thought might interest you all.
I have an Airport Express connected to a SONOS speaker, and I was streaming my iPhone's audio to said speaker using Airplay.
I suddenly noticed that the VoiceOver output was coming from my SONOS speaker instead of my iPhone, but the standard VoiceOver sounds were audible from the iPhone's speaker (clicks, beeps, rotor sounds, etc).
By complete chance, I noticed that the Audio Ducking setting in my VoiceOver rotor was turned off.
When Audio Ducking was turned on again, the VoiceOver output immediately returned to my iPhone, and my music came from the SONOS speaker.
So the moral of the story is, if you want VoiceOver output from your iPhone's speaker while using Airplay, ensure that Audio Ducking is turned on.
Note - Audio Ducking is turned on by default, and it is only present in the VoiceOver rotor if you opt to have it there in the rotor settings.
I had Audio Ducking included in my rotor settings as sometimes VoiceOver ducks my music when it is speaking, but the music does not return to normal volume after it is finished speaking.
I hope the above information is of benefit to someone.