MC uses DLNA to connect to Sonos speakers. Barring any firewall issues, the speakers should show up in Playing Now. Each Sonos speaker shows up as a separate entry. You can use either the generic DLNA server or create a separate one for the Sonos speakers. The main thing you need to pay attention to in the DLNA server in whether the original format is used or when it is transcoded to another format, like MP3. DLNA settings are under Tools - Options - Media Network. Right click on the speaker to associate a specific DLNA server to that speaker.
When using MC, the Sonos grouping of speakers is not used. Each speaker is a separate speaker. You can link the speakers within MC, but the sound will not be totally in sync. If you can hear multiple speakers at the same time, you will probably want to use the Sonos app rather than MC.
To use the Sonos app for playback, you need to create your own library within the Sonos app. You basically tell the app to build a library by pointing it to the music folders on you PC. The Sonos app accesses the files on your PC for playback. In that mode, your speakers will be synced.
Give it a try both ways.