If you don't already have a Port you might want to wait until WiiM's DLNA Cast feature eventually appears. In theory it ought to be able to stream to the desired Sonos unit directly. That would avoid all the hassle of converting to analog, sending the signal via Sonos Line-In, and converting back to digital.
DLNA can be a bit finicky in general but in the past I've used DLNA successfully to cast to Sonos players from apps like JPLAY and BubbleUPnP.