Is there anyway to use Airplay 2 Speakers as an audio out? My whole setup is sonos, but I don't see how to connect them to the Roku Express 4K...
No, the Airplay functionality on Roku doesnt support that:
https://support.roku.com/article/360057488733
Apple AirPlay lets you stream, control, and share content from your iPhone, iPad, or Mac to your compatible Roku streaming device. You can watch movies, TV shows, and family videos, listen to popular music, or browse vacation photos. You can even mirror exactly what you see on your Apple device – all while enjoying the big-screen experience from the comfort of your couch.
Apple HomeKit provides an easy and secure way for you to use Siri and the Home app on your iPhone, iPad, or Mac to control your compatible Roku device. Or, you can use scenes to group your Roku device with other HomeKit-enabled devices to control an entire room at once.
I would love to see this as well as I have just ordered a higher end Projector and would find it much easier to plug a Roku into the HDMI port than an Apple TV which is bigger and requires a power adaper