Perhaps you might want to read the regulations directly? Since Sonos doesn’t make their own in ceiling speakers, but instead sell another companies devices, it’s unlikely that Sonos tracks that data. 
 

That being said, I’ve never heard of any such requirement for essentially low power devices, but would imagine that if such were necessary, it would be in the manual for such devices, as part of federal regulations. 

If no Fed Reg…you might check with your local city hall that issues construction licenses or a local electrical installer to determine if any regs exist for in-ceiling/in-wall speaker installations.

If there are such local regs and you don’t follow them it could result in a negative insurance claim with your home owners policy in the event of fire caused by the installation. 

This is really picking at straws for worse case scenario….but since you asked…you might investigate for your own peace-of-mind.

Most libraries have a copy of the current NFPA code book. Some will loan it but most require you read it there.

I’d probably pay a licenced installer for that answer rather than wading into the code book, done that digging for past projects and it is painful reading.

I agree with Airgetlam, that Sonos would mention federal regulation requirements in their manual.

There is a lot of mention for these fire hoods from UK users. It seems they are a requirement there.

In my case the corpus of the architectural speakers would be embedded into blow-in insulation within the ceiling. Is this causing any trouble or a need to have some kind of an encasing for the speaker?

The challenge there is Sonos doesn’t make in ceiling speakers, they’re made by a company called Sonance, which Sonos has a marketing agreement with. If you’re looking for additional information, your best bet would be to contact Sonance directly. Sonance would be the ones in control over what is in the manual for those speakers. 

I’ve never seen an inceiling/inwall speaker come with any sort of enclosure for the purpose of fire protection.  I’ve seen some with enclosures for acoustical reasons, but not safety.  Can’t say I’ve read the safety section of the manuals that much, so perhaps they cover themselves by stating you need to follow local codes.

I could be completely wrong about this, but I would think the electrical wiring would be just as much if not more of a concern the speaker itself.  Which is why you definitely want to use in wall rated wiring.

It also seems that local codes tend to follow national codes in order to avoid confusion.  The main exception seems to be when the local environment requires some modification.  For example, there’s little need for freeze protection in plumbing where I live, it’s mostly an unnecessary cost.  Not so as you go further north.

The blown-in insulation issue depends on the speaker design, closed back speakers are fine, ones with open backs need protection.

Personally if I was putting speakers above the ceiling I’d put a cover over them. It offers a bit of protection from the attic heat and other contaminants that accumulate up there.

Many styles but I like the rigid ones best:

https://smile.amazon.com/Bogen-BG-RE84-Recessed-Ceiling-Enclosure/dp/B00FMH2GZS/ref=sr_1_11