I have created an Athom App that makes use of node-sonos-http-api to send out speech from your flow. Setup up a node-sonos, install the app, point the settings to your node-sonos instans. Then you just add the Text to Speech in your flow and wola, your sonos will start talking.
Since the app needs you to install node-sonos on a separat instance the app will not be approved for the official app store (tried and got declined).
But for you who want, you can download it here and run it:
Hope you like it