Some way to get the signal to the line in on the Era 100s. Which likely means some sort of cable connection to a Sonos line in device connected to the Era 100s. Note that all Sonos analog line ins have a minimum of a 75ms delay, so if lip sync bothers you, it could be a problem. If the projector has Bluetooth, the Era 100s generally are accessible for stereo that way, too.

Sonos doesn’t really make a solution for projectors, perhaps unfortunately.