I was looking for instructions as to how to use my Sonos speaks as my Nest Doorbell chime but didn’t find anything that explained how to do it.  After various searches, I found bits and pieces of how to get my Nest Doorbell to announce over my Sonos system.  The important point is that it can be done and it does work. 

There are a number of steps and I’m not sure which are required, but here’s what I did:

1. From Google Home app, add Sonos to Services. Go to Settings screen. Select “works with Google” under services. 
2. From Google Home App, go to Settings and select the Nest Doorbell.  Click on Doorbell and in the Features section, make sure that Visitor Announcements is active. 
3. From Sonos app, go to Settings screen and click “Manage”.  Find “Voice Assistants” and add Google Assistant. Once added, click on Google Assistant and click “add to another product”. Add the Sonos speakers you want to announce. 
4. From Sonos app, go to Settings screen and click on your target Sonos device.  Scroll to Voice section and add Google Assistant. 
5. During the process, you will be required to install the Google Assistant app on your mobile phone/tablet.  From Google Assistant app, go to Settings screen. Confirm that Sonos is linked in the Home Controls section.  Then click “Devices” to manage the devices linked to the assistant.  Click “add devices” and add the target Sonos speaker. 
6. I’m not sure if microphone needs to be physically turned on on the Sonos unit for setup to work. But I have confirmed that it can be turned off after configuration and announcements still happen. Mic controls can’t be controlled from the app (even though you can see the mic status).  There is a physical button on the device that you have to touch. 

Hope this helps others out there.  If you have mods to this setup, feel free to post.