Yes you can, but in my own tests of this Amazon ‘everywhere’ feature, I found that the audio-out from the Firestick to the Sonos Arc was way out of sync, compared to the other playing echo products, but that said, my Firestick had a TV in-between the Firestick 4K and the Sonos speaker.

I’ve heard there was a way to change the delay setting to make it sync. Please let me know if you find this to be true 

The Amazon sync options are only for use with Bluetooth connected speakers to the Amazon echo device - so (just as an example) a Sonos Move, or Roam, which maybe BT paired with an echo device. Those type of devices do allow for audio sync adjustment. See attached screenshot, but don’t think it can be done with a Sonos Arc connected to a Firestick.

@jag7707 
What you can do, is Bluetooth pair a Sonos Roam to an echo device that’s included in the ‘Everywhere’ Group (such as an echo Dot for example) and play the audio to all (you will (should) find the Roam Play’s pretty well in sync with all Amazon players in this mode) although there is always the sync slider-bar feature in the Alexa App, just as a fallback, but you most probably won’t need it. 

As the Sonos Roam can also simultaneously ‘act’ as a Bluetooth line-in (over WiFi) to a Sonos System, if you open the S2 Sonos App after starting the playback on the ‘Everywhere’ Group, you can go onto ’group’ the Roam in the App to any other Sonos devices in the Household, so you could group it with the Sonos Arc for example …and all (Sonos and Echo devices) will (should) then play the audio stream in sync - but you will need a Sonos Roam to achieve this. 

Thank you for the thoughtful response. That’s too messy. My dream would be Arc could be used as an Alexa speaker within a group, but I know that’s not possible.

Much the same as you can’t use an Alexa, or frankly, any other company’s,  speaker as another Sonos speaker. Both companies want to keep you within their own ecosystem.