You will only get stereo from Airplay.  In order to get DD 5.1 or Atmos, you need an HDMI-ARC source connected to the Sonos Beam via an HDMI cable.  

If you use the MacBook HDMI Out you can add the HDFury Arcana, Arcana2-VRR or VRRoom to the system.

You connect the MacBook HDMI Out to an Input on the HDFury then connect an HDMI cable from the HDFury to the Projector plus a separate HDMI cable to the SONOS Soundbar, that allows the full Video and Audio capabilities of your Source device/content stream.

Joe

The Media Factory 

Yes, you can connect your home theater surround system to your iPad or MacBook using AirPlay 2. To do this, make sure your Beam and Era 100 speakers are compatible with AirPlay 2. Connect the speakers to the same Wi-Fi network as your iPad or MacBook. Then, on your iPad or MacBook, select the AirPlay icon and choose the Beam and Era 100 speakers as the output devices. This will allow you to stream audio from your device to the home theater system while streaming video to the wired projector.

https://www.projectoreye.com/does-projector-work-on-mac/

To my knowledge, this previous post won’t work. The best that an AirPlay 2 signal can carry to a Sonos device is a stereo signal, as stated above by @jgatie and not a Dolby Digital , or Atmos signal. This is why you need to send to an Apple TV device, which would be connected to the TV set/projector and then the Sonos with HDMI cables, to carry the ARC/eARC signal to the Sonos.

The Era 300 speakers wouldn’t be seen in the AirPlay 2 selector, since they’re a “child” to the “adult” of the Beam. They connect (and get IP addresses) through the Beam from your router, but won’t show up as AirPlay 2 targets. That’s if they’re set up as surrounds.

If, however, they’re not surrounds, but instead just a second “room” grouped with the Beam, then they’d show up, as they’re no longer “children” of the Beam, but instead “peers”.