I like using the room control driver event "Listen device selected" as a "music is playing here now" trigger.
If you're specifically only using Sonos, look deeper into the Sonos network driver events/actions, since that is what directly controls Sonos matrix. On the output tab of the network driver events, "When audio [int] changed" directly correlates to the Sonos room joins, and is bidirectional with C4 at this point. "Audio x" is determined by which rooms are connected to the network driver audio zones. When I program stuff like this I like to have a list of each Sonos zone written down and the corresponding C4 Sonos network audio number that's associated with it.