The Arc does not have an optical port per say, it has an HDMI-ARC port only.  However, it does come with an adapter to convert optical to HDM-ARC essentially.  If you already have an Arc and a TV though, I would recommend getting a streaming box that is chromecast capable, and running thre audio through there, rather than having to disconnect your Arc from the TV, and plug in this device every time you want to use it.

What content/sources do you want to play by this method, and using what device?  I can’t think of too many audio sources for which there wouldn’t be a better approach.  But I could well be missing something.

I would like to connect the chromecast audio to the Sonos via optical to use it as a high res streamer endpoint that’s Roon Ready.

I know Sonos can be seen as and endpoint for Roon but it’s a lower res that Chromecast audio…

If you end up playing through a Sonos speaker I don’t think you will get any improvement in audio quality.