Currently, the soundbar volume is controlled by the volume buttons on your smartphone only when you are in the Sonos app. The moment you leave the Sonos app screen(this does not mean quitting the Sonos app), you cannot control the soundbar volume with the volume buttons on your smartphone. What bothers me is that I have to stay in the Sonos app all the time to control the soundbar volume. The moment I leave the app to do something else on my smartphone(for example, leaving the sonos app to text someone, or leaving the Sonos app to browse the internet for a while), I cannot control the soundbar volume. I have to come back into the app screen.
P.S. Again, this assumes that you have left the Sonos app screen for a while, not that you quit the Sonos app.