Google Play is an Android app store. Did you mean Google Assistant?

Google Play Music is no longer available as Google have discontinued it. 

YouTube Music is being offered as a replacement for GPM, and you can easily transfer your uploaded music library from the old to the new platform.

Note that while Google Play Music offered native Sonos casting integration (though this wasn’t Chromecast), YouTube Music no longer offers this option, and as Sonos devices unfortunately don’t support Chromecast, you will have to either AirPlay from your Apple device if you have one, or use the Sonos app or voice commands (if they happen to work) to control YouTube Music.