Sonos Architectural ceiling in Bathroom?

I'd skip the Sonos ones and get some designed for damp locations if you see any condensation  on the walls or ceiling. Maybe look at in-wall Sonos ones that could be mounted below the condensation zone.

A quick look showed no moisture ratings  ut I could have missed them.

Just as a note, the speakers in question are sold by Sonos, but not designed or manufactured by Sonos, but a similarly named company called Sonance (no relation, other than marketing).  You may want to do a web search for commentary about ‘Sonance Architectural’ and see what is pulled up. 

Sonos in-ceiling speakers are not designed for damp or moist environments. You could install Sonos Outdoor speakers but that defeats the purpose of having invisible speakers in the ceiling.

You don’t NEED to use Sonos or Sonance speakers. Especially since you are just using a single pair.

Here is a good solution from Polk. They come in a 6.5 and an 8”:

https://www.crutchfield.com/p_107RC60I/Polk-Audio-RC60i.html?awcr=625514282818&awdv=c&awnw=g&awug=9004441&awkw=kwd-300117514691&awmt=p&awat=&gad_source=1&gad_campaignid=173692163&gclid=EAIaIQobChMI5ImFw4blkAMVhlhHAR1VsgDVEAAYASAAEgLbtvD_BwE