Apple TV 4K with Atmos using hdmi splitter?

No.  The Sonos Arc can’t accept a normal HDMI connection from a source like an Apple TV, it must receive audio through audio return channel, ARC or eARC.  The device you would need is the Arcana, which is currently in preorder.  The diagrams on the ‘Getting Started’ tab do a good job of illustrating how you would use it.   Your case sounds like ‘One Source Only’

Thanks a lot Melvimbe, you are right arcana it's what I need.

Hi @Josena, Thanks for reaching out and welcome to the community. I would agree with @melvimbe on this concern. and the Arcana is the one you can check that might help in your concern.

Let us know how you get on with the advice above.

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