Since Chirp, to my knowledge, hasn’t added themselves to the Sonos ecosystem, you’ve got two choices, either via a line in connection between the Mac and an appropriate Sonos device, or AirPlay 2 between the Mac and an appropriate Sonos device that can receive AirPlay 2. 

Since it’s an iPhone, I guess I’ll have to wait until Sonos adds Chirp.

You have no Sonos speakers that are AirPlay 2 capable? I frequently use AirPlay 2 on either my iPhone, or my iPad, to get content into my Sonos ecosystem. Today, it was a pre-season baseball game. 

And I see I didn’t add the fact that AirPlay 2 exists on iOS devices in my original answer, my apologies. 

By the way, if you want to nudge the folks at Chirp, send them the link to the Sonos partners page. They can read up on how to add themselves to the ecosystem.