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.
This isn't really a solution, but we experience similar and do the following:
* Using the SONOS app, adjust the volume to something decently in the middle that works. But we do this in conjunction with the Bravia and the Roku remote to get the levels around 50 about right for normal TV. This way, whatever we switch to is generally in the ballpark and not crazy loud or soft when you first switch to it. It's not like I've taken a volume meter to it to see the differences, but it seems like there's some kind of difference just coming from the source, so I'm not sure there's anything we can do about it. (Unless you found a way to put in some other kind of equalizer box in between, but that seems like crazy overkill. I think we just have to live with it.)