I'm surprised Amazon still hasn't added music syncing across some/all Alexa devices. It's not hard, I can think of three ways of coding it without even trying - after all, Sonos has been doing it for years.
Music sync would be THE killer feature that would sell millions more Echo devices.
I wonder what the holdup is? Maybe Sonos has a patent and they won't sell rights at a reasonable fee?
I wish so hard every day I could have whole house music via Alexa.