The Arc will perform an audio up-mix of the stereo signal to create simulated surround sound. This means you’ll still hear most audio out of the Arc, but the surrounds will play what is determined to be ambient audio.

The other option is to use a different streaming box that supports 5.1 audio. Make sure your TV supports 5.1 pass through too.

Getting a better streaming box is my suggestion as well.  Replacing your streaming box is surely the cheaper option.

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Hi thank you all for your reply’s. Im using a Amiko A11 BLU 4K UHD Android Mytv3 box and any movie I watch it says stereo in the Sonos app. I have also tried the box in my summerhouse where I have the same set up expect in there I have a Sonos playbase instead of the arc. Same thing.