Just curious...what "different brand" speakers have Sonos built in?
None, they're a walled garden; they've upped their game considerably lately with the Play:1 and especially the Play:5, both multi-amp'd with room correction built in, and of course can integrate either with their sub and/or playbar for multi-channel AV systems.
Currently the only way to use third-party speakers is via their Connect at $350, which does basically what the CCA does (plus an analog input) for $35. So, an issue for Sonos, except that the Connect isn't their cash cow, by any means.
But, if the Goggle Cast multiroom proves to work well, becomes a standard (it already has on the streaming provider side), and is built into high quality powered speakers from well known speaker companies (only LG, Sony and Denon so far), Sonos will finally have some real competition in the wireless multi-room market they invented. So far, virtually all the multi-room competition has either failed, hasn't offered nearly the feature set (Bose, Polk, etc. multi-room speakers don't sell nearly as well and offer three or four services vs 60 or so from Sonos), or has been targeted at the tiny "high end" market.
Sonos has been hugely successful due their building-block approach, rock solid network reliability, high sound quality, simplicity, and great reviews. The ability to start casting to a new pair of speakers in five minutes sells a lot of non-technical consumers on the system. If Google's partners start offering that same experience, with perhaps even better SQ, then Sonos will have to start allowing third party licensing, lower costs, etc. Competition is good.