I’ve seen issues like this on the forum but not exactly like mine. I bought a Beam a few weeks ago, and connected to a few years old Sony Bravia with HDMI ARC. I mainly stream to the TV through a Roku Streaming Stick+. I connected the TV in the “right way” with Audio System settings and the Beam is being controlled through the Roku or the TV remotes.
Issue is that audio received from Roku on channels such as Netflix needs to be really high in the 50-80%, while audio from the Sonos app or when using direct audio from the TV with an app like YouTube or Amazon Prime needs to be around 25-35% To sounds normal in this room.
Also it appears that even though the is on Dolby Digital, it may be using Stereo. And when using DD+ no audio is coming out.