It seems the S70 (with wi-fi) could operate IP-controlled music servers if there are widgets designed for them. But I don't believe there is one for Sonos, and Sonos is frustratingly reluctant to open up their system beyond their own controllers. (Personally, I think it's hurting their competitiveness now, as more and more people are using universal remotes and components that won't play along seem kind of 1990s.)

Controlling your volume that way is really not an ideal solution.  It's better to set the Sonos to "Fixed" volume and use the receiver's volume control.