Best answer by jgatie
All you had to do was group the speakers using the Sonos app, ask Alexa to play to any one speaker in the group and the whole group would play. At the current time, grouping 3rd party speakers is not in the Alexa API, but it is coming early next year. How much Sonos will be able to utilize of the new functions we have yet to see.
To the OP, no need to buy anything else, simply see this link on how to add Alexa control to your Sonos system: