Let’s start with the basics - and assuming you want a home theatre setup:

Set them as surrounds bonded to your Arc so they are all part of the same room. 

Then check: what source are you playing, and is it sending an Atmos signal to the Arc? Then check in the app: is it showing as an Atmos signal? If not you’ll need to change the audio settings on your tv. 

Follow the instructions in this article to add the Era 300s as surrounds to the Arc:

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

What model TV do you have? What exactly is your Dolby Atmos source?

I have a Panasonic recent model TV that supports dolby atmos on HDMI2. I stream all my services through an Amazon Cube. I don’t think the atmos is getting through to the arc as it’s not shown anywhere. I will try your suggestions although I think I have tried them before. Thanks for replying.

Have you adjusted the TV settings so it can pass through Dolby Atmos audio? Also be sure to set the Surround Sound setting on the Cube to Best Available.

I have now set my cube sound settings to ‘Best Available’ and success. It played a demo of a waterfall in Dolby Atmos using the Arc & 2 x Era300’s + Sub. It sounded great! However, the Era300 speakers only emit sound now when I play a Dolby Atmos film etc. Is that the way it’s supposed to work?

Many thanks for your good advice.

You should hear audio from the Era 300s when playing Dolby Atmos and 5.1 audio. You can also hear audio from the Era 300s when playing higher quality stereo audio. But keep in mind that you won’t always hear audio from the surround speakers especially during quiet scenes. The audio from the surrounds will usually be the musical score, action that is behind you, or ambient sounds. Try finding an action film in 5.1 to test.

Hi, I’ve now tried a 5.1 action film and it did come through as Atmos also. So your advice that the Era 300’s would play sound for Dolby Atmos &  some 5.1 films is correct. Thank you very much for your advice. It was a great help to me.