HomePod as speaker for iPhone and smart TV

HomePod is not a universal Bluetooth speaker. It works with Apple's AirPlay protocol. You can AirPlay from iOS devices, Apple TV, or iTunes running on Mac or Windows. See How to AirPlay audio - Apple Support.

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See https://9to5mac.com/2019/02/11/samsung-smart-tvs-will-soon-add-itunes-movies-and-tv-shows-apps-airplay-2/. It seems Samsung are adding AirPlay support to some of their models. Whether or not yours is one of them is another matter.

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Although I’m not entirely sure how it will work on certain Smart TVs with this feature, I think it means that you will only be able to airplay to the TV and not from the TV to HomePod. The same way we can mirror content from our iOS devices to Apple TV, but cannot send the audio output from said content to HomePod.

We can currently send audio to HomePod from Apple TV if playing directly from Apple TV.

I’m just not sure about this and Samsung aren’t the only brand participating by the way.

I am afraid you are probably correct in your assessment of AirPlay 2 enabled TVs. I plan to buy a Sony A9G which supports AirPlay 2 and I'm looking for a way to direct TV audio from the cable box to the HomePods, not just from the Apple TV. The TV itself has speakers so I can use them with the cable box but it would really round out the HomePod solution if there was a way to play to the HomePods from the TV using AirPlay.

I’d like this option as well.