You may want to check with the manufacturer of the ceiling speakers you’ve purchased. 

They actually are the in ceiling speakers from Sonos - Sonance.

Sorry, Sonos doesn’t make Sonance speakers, they just sell them in their marketplace. I think you may want to contact the folks at Sonance at https://www.sonance.com/, you’d likely get much more knowledgeable responses from them. It’s possible that some of the Sonos folks may have some experience, but since Sonos is just a reseller, I wouldn’t count on it. 

If your ceiling speakers are “infinite baffle” designs, they are supposed to be installed without a box.  However, many use the box in order to isolate the sound from other rooms, and put up with the slight deterioration in mid-range tones that results.  

Well maybe I am wrong but I thought Sonos worked in collaboration with Sonance and have their joint range of in ceiling and in wall speakers. 
 

I’d also assume that because Sonos its name is on the speaker, they would also be able to tell on how to correctly install. 
 

I do appreciate your response but surely somebody on this forum -or somebody from Sonos- would be able to answer this fairly simple question? 
 

To be sure we are talking about the same speakers;

https://www.sonos.com/shop/ceiling-speaker-pair.html

This is a user forum. Sonos does answer questions, but it would be faster just to call them. If you do, please post the answer here for futire reference.

I followed your advice and Sonos responded that there is no need for a back box. You can build them straight in your ceiling and should be designed to be mounted this way. 

It may be necessary anyway, so I would also recommend checking with a contractor. 
 

We are having a house build and I also asked about the Sonance speakers. 
 

Of course regulations may vary from country to country but at least in Denmark, there is a membrane between the ceiling and the attic to protect for moisture etc. That will likely need to be breached to install the speakers and if that is the case, Danish regulations require installation of a back box which is then sealed tight to the membrane. 

That is the case for us so we need backboxes. 
 

In any event, there are some advantages to it acoustically as you limit spill-over of sound between rooms and it will likely help the low ends of the speaker as well.