https://www.cnet.com/news/google-announces-assistant-on-sonos-speakers-coming-in-2019-hands-on-ces/

Sonos says that seven streaming services will be active at launch, but that the goal is that Google Assistant will control every one of the more than 100 services that Sonos supports. The company demonstrated Pandora and Google Play Music at the demo, but was hesitant to list the others.

Sonos has confirmed to CNET that you will not be able to toggle on the fly between Alexa and Google Assistant. Instead you'll have to choose one or the other during setup. You can, however, have both assistants in one whole-home system and make them interoperable. That is, you'll be able to use Google Assistant to control Sonos speakers configured with Alexa, and nonsmart Sonos speakers, with a future update.