HomePod Mini default audio output

See Set up home theater audio with HomePod and Apple TV 4K - Apple Support for details on if and how you can allocate default speakers. If it isn't possible to use your HomePod (i.e. if a mini) as the default speakers then it should still be possible to select your HomePod as an AirPlay target instead of, or as well as, your TV's speakers.

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No, that option is currently only available for the original HomePod. Use Feedback - HomePod - Apple if you want to suggest it should be otherwise.

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I am using an HP Envey 34-a051 as my tv for my Apple TV. I can't use the Apple TV to adjust sound on the monitor and it appears HomePod is not available. Can the default speaker be changed or am I limited to tv speakers or HomePod?