In case anyone was interested, the audio lag when watching videos / youtube / music using Airplay from a Mac/ OSX has been resolved with Sonos S2. You can also send airplay to multiple Sonos speakers (although this was possible previously).
When watching Youtube / any video content, select one of the Sonos speakers from the main apple sound menu (speaker top right in OSX). You will only be able to select a single speaker here, but if you go into the Sonos App (on the desktop or iphone / android) you can then group speakers with the original speaker selected on the mac, and set the independent speaker volumes etc.