Assuming a fairly poor setup 5.1 system using the Sonos instead of a proper AV receiver and speakers, you should put the surround speaker to the left and right of the couch on the walls for fairly decent imaging. It's not best, but gets the surround sound to the sides and opens the space up some.
You will have bigger issues with most projectors as they rarely have optical audio output and if they do, it may not be in full surround sound. Though, I would seriously consider the BenQ X3000i if it fits your space as I believe that model supports a fair bit of connectivity. Unlike a TV, do NOT assume a projector with have the proper connectivity your speaker bar needs.
That said, you can pick up a HDMI audio extractor with Toslink digital optical audio output for under 50 bucks on Amazon.
At the end of the day, I would put in an AV receiver and proper speakers. Cheap in-wall speakers at $40 each will sound a great deal better than the Sonos.