VLC media player has had this functionality for many, many years (very handy to do multi-room audio at parties for no cost!). See (dated 2005):
The page linked above demonstrates all of the three required criteria for prior art but does takes some reading to fathom. The first page I found that walks you ...
Does this count:
It mentions a mesh network between separate Sonos devices.
The date published mentions 05/13/2004, but the document has been updated in 01/07/2013.
The picture of the mesh network:
is dated as modified on 17 July ...