Terrific article from iMore’s Lory Gil. Long story short:

• On Apple TV, go to home screen (this next step only works from the home screen).
• On Apple TV remote, hold down pause button for 3 seconds or so.
• When AirPlay menu appears, select your HomePod. This will route Apple TV audio to HomePod.

Now go play a movie. The audio should be coming out of your HomePod. If you are watching in front of your TV, this isn’t really ideal, but if you are moving around, perhaps working in the kitchen with a long view of the TV screen, this can be terrifically convenient.

Now that audio is piped into your HomePod, you can say things to HomePod Siri like:

• Volume 60 (that sets the volume to 60%)
• Pause (pauses the video playback)
• Go back 20 seconds

Good stuff.