You can only add two surround speakers to a Sonos home theater setup. So you would use a Sonos Amp to power the pair in the back and use as surrounds with the Arc. The front pair can either remain unused or you can get another Amp to power those speakers and use for streaming music on their own or grouped with the Arc/surround speakers setup. NOTE: You do NOT want to group the front speakers together with the Arc/surround speakers when playing TV audio from the Arc because it will result in a delay/echo effect.

that’s what I thought, thank you.

The problem with using the installed speakers as Surround channels is that then messes up Immersive audio if you are aiming to support Atmos!

Where are the installed speakers placed relative to your seating and is Atmos something you want to support?

SONOS is not designed to directly deal with this speaker arrangement. There are some options, each with positive and negative aspects.

• Ignore the second pair of speakers. Depending on their placement relative to your listening position, the second pair may distract from the surround effect. Surround speakers should not be located in front of the listener.
• Install a switch to disconnect the front ceiling speakers from AMP while watching TV. This will not mute ARC. You’ll need to setup and try this arrangement before deciding if this is good or bad. This is not the friendliest user experience.
• If ARC running along with the four ceiling speakers does not sound right, install a second AMP that will be used only for music and switch the ceiling speakers to this AMP for music listening. Again, not the friendliest user experience.
• Rather than fussing with a second AMP and the switching both pairs of ceiling speakers, you can switch one pair and use the controller App to somewhat adjust the relative Volume between front and rear. This is not a perfect scheme, your mileage will vary and, while avoiding the second AMP, there are still some user experience considerations.

The user experience considerations will affect each household differently. 

why would the in ceiling wired speakers connected to a Sonos amp mess up Dolby Atmos? The speakers I would use would be behind our heads in the ceiling. What would you suggest to support Atmos? Installing new speakers????

why would the in ceiling wired speakers connected to a Sonos amp mess up Dolby Atmos? The speakers I would use would be behind our heads in the ceiling. What would you suggest to support Atmos? Installing new speakers????

Are all four speakers located behind the listeners?

Essentially. There are two rows of seating and the speakers are in ceiling above each row. Based on everybody's feedback, I would activate the “2nd row” speakers as surround since we mostly sit in the front row. 

Immersive audio (Atmos) requires vertical separation between the ‘bed’ 5.1/7.1 speaker layer and the immersive overhead speakers, if your surround channels and immersive channels are all coming from the ceiling you can forget about any semblance of overhead effects.

Where you are using side or rear surround channels in an Atmos setup they have to be set at, or close to, your seated ear level.

Ahh makes sense. So to get the Atmos effect, I would need to put Sonos era speakers on stands at about head level (perhaps just behind and above the back of the chairs. Is that correct? 

Hi

To be clear the overhead speakers would not mess up Dolby Atmos.  Prior to the Era 300’s…Sonos One’s or any Sonos standalone speaker pair (except Move, Roam or Sonos Play 5-gen1) were used as surrounds with the Arc.

The Arc still delivered Dolby Atmos sound but it wasn’t entirely as immersive pre-introduction of the Era 300 series. I used Sonos One’s with my Arc and was able to enjoy Dolby Atmos. I have since replaced them with Era 300’s.

However, Dolby Atmos or not in-ceiling speakers are not the go-to speaker for surround effect. The listening position is critical reference the seating placement. IMO, if rear speaker invisibility is of utmost importance I’d opt for Sonos In-wall speakers. The Dolby Atmos effect would still not be the same as with Era 300’s used as rears; but no worse than using Sonos One’s.

As you moved into a house with the two rows of in-ceiling speakers I suspect the previous owner used a traditional surround setup with separate left/center/right front speakers, sub and amp which yielded 7.1 surround. Sonos is not designed to duplicate that type of setup. You might want to consider a traditional 7.1 surround setup and possibly consult a professional installer.

Downward firing Surrounds means no separation from the height channels (in-ceiling or up firing), you are wasting your money trying to achieve Atmos if your Surround and Rear channels are in-ceiling.

Assuming your are creating a long term solution I’d outline what you want to achieve, the available budget, the room seating layout and ignore any preinstalled speakers, I’d avoid a long term compromise to make use of badly placed ceiling speakers.