I don't understand why this is limited to the watch. Sure - its handy to have it on your wrist for immediate access, but expanding it to support phones would really help increase the number of people that can use it. Even if you have an apple watch, you need to know someone else that also has an apple watch. I can see this being useful for family stuff but none of my immediate family has an Apple watch.
Could easily be a widget on the phone. And while you might not hear the audio if the phone is in your pocket, you would get a vibration, and it would play through your headphones.
But then where's the incentive for someone to otherwise buy a watch?