Apple may have discontinued the original HomePod earlier this year, but fortunately the company is still supporting it with useful new features. If you have a HomePod and picked up the new Apple TV 4K, your speaker will make for a better home theater companion than it did before. As noticed by MacRumors, the latest Apple TV 4K supports the ARC (Audio Return Channel) and eARC (enhanced ARC) standards over HDMI.
What this means in practice is that the Apple TV 4K can wirelessly send all audio from your television to a HomePod (or stereo pair of HomePod speakers). Previously, the HomePod could work as an audio output device for the Apple TV — but now, the Apple TV 4K can pass all audio from a TV wirelessly to the HomePod. Your cable box, console, or other set-top box can all then have their audio played back through the HomePod, making it a much better home theater companion.
To enable this, just go to the audio output settings on your Apple TV — from there, you can make sure the HomePod is selected as your output source. And that's also where you'll enable the Audio Return Channel setting (which is, somewhat strangely, marked as in beta). Provided your TV (or receiver!) supports eARC, you'll be good to go.
While the HomePod didn't work great as an audio output device when it first launched, Apple has increasingly made it more useful when pairing it up with an Apple TV. Last fall, Apple officially brought Dolby Atmos support to the HomePod, though the feature doesn't work with the smaller HomePod mini speakers. The HomePod mini also won't work with these new ARC-based features. That's not a huge surprise, as the small speaker is significantly quieter than the original HomePod, making it not terribly appropriate for a home theater setup.