I periodically have this problem with my sonos setup, usually my MOVE speakers (there is more than one problem). But in any event, with the MOVEs, I think I simply close MC and let it restart and find the speakers again. This is not frequent, but not terribly unusual. If need be, I'd just remove them and let MC do a complete reinstall. At worst reset the speaker and reinstall on the SONOS app. I don't think it is likely a "setting." My best guess.
P.S. I also have a SONOS 5 speaker tv sound system (using ARC) and it works well with MC
P.S.S: One more thing to try. I have found that JRemote (Iphone app) can (initially) sometimes work more reliably than working directly off MC on the desktop. Once JRemote gets things started, MC on the desktop works fine. I have no idea why that is sometimes the case, but it is part of my trial and error solutions. (JUst to get the speakers working initially)