You can airplay to them, it will be limited to stereo. But the minis are limited to stereo even with used in "theater mode"
You will probably have to re-connect to airplay every time you wake the aTV, and it might also disconnect while it's on. Shouldn't happen in the middle of watching something, but might while you're in the menu between shows.
You can connect to airplay from the control center, and possibly with siri, so you don't have to go into the settings app every time.
You may be able to use default audio, but I interpret the instructions the same way as Darth, that it only works with the 4k, but the names and models have shifted over time, so it might work.
Default audio always stay connected, and if you have full-size homepods, it will give you "surround" and "ATMOS" or at least as much of those as you can have with only having speakers in the front.
Both modes will work better with a solid wifi signal. If it's possible in your setup, plugging the aTV into ethernet can help with this a bit, as it takes some of the bandwidth off your wifi.
Use a HomePod, HomePod mini, or a stereo pair with your Apple TV 4K to create a theater experience with Dolby Atmos or surround sound right in your home.
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