Best answer by Ken_Griffiths
With the current IFTTT Sonos Applets (actions) you can only select one Sonos Room and also the other rooms that are already 'grouped' with it. So you need to group the rooms together first. You can obviously do that manually in the Sonos App, but...
There are possibly some workarounds to this with other IFTTT Applet actions.
Whilst researching your issue, one device/applet I discovered that apparently can group your speakers for you beforehand, is the Harmony Hub/Remote and their IFTTT Applet.
If you have such a Harmony Hub or Remote, you can do this by creating/running a 'Harmony Activity' in the My Harmony App, which will 'Group' your Sonos speakers together, but then not play a playlist ...The created Harmony 'Activity' can then be set as an Harmony Applet action in their Applet to trigger from your Nest Protect 'emergency' trigger etc. .. see my screenshot of the Harmony Applet trigger.
There maybe other devices and Applets that will do this, but not looked any further at this point in time..
I have not tried it myself, I don’t have a Nest Protect etc; but suspect it would work. Otherwise you may have to manually group your speakers, until Sonos extend their IFTTT Applets to somehow include grouping or at least all Sonos Speakers, which I’m reasonably hopeful might 'perhaps' happen at some point in the future.
Hope that helps ?