I have an Roku Ultra (4670x) connected to my Samsung TV via HDMI. The TV is connected to a Sonos Beam (gen 1) via SPDIF. The Sonos shows all audio sent when using the Roku Ultra as Stereo, yet if I play the same app from the TV the Sonos shows the audio as DD. How do I get the Roku to output DD same as the TV does?
If you're using any of the Volume Modes (leveling, night) you'll only get stereo. There could be something else going on, but that's the first thing to check. Press * while a video is playing and look for the Volume Modes somewhere in the menu.