Though many people have asked for this it is not possible. This might be because the way speakers are commanded is very different for both configutaions - in a homecinema set up the surrounds are commanded by the soundbar (or Amp) via a dedicated 5Ghz link, while when set up in a "room” of their own they govern themselves via your wifi or Sonosnet. The system would have to remember both configurations and preferably the Trueplay details (that are lost every time you change the set up) for both configurations too. So this might not be very easy for Sonos to program.
@pabvalles I’m curious what your room setup would have been to accommodate this, had it been feasible.
I’ve just ordered two era 300s, with the intention of setting them up as surround rear additions for my sub and arc. I also ordered a port to bring my turntable into the system. Ideally, my home theatre would have doubled as the ideal place to listen to vinyl, but it sounds like my setup may have to become a dedicated home theater rather than a “primary” or optimized music space.
I assume your 300s are setup as rear surrounds as well. would you be willing to describe your setup?
I really like my soundbar and surrounds for music, I switch from the default Ambient mode to Full and get a lot more and full range sound from the surrounds.