THANK YOU! I was going crazy with this. Using an Optical cable with Vizio sound bar and 4k Vizio tv. No matter what I did, I kept getting Stereo instead of Dolby Audio. Changed Streaming audio format to Auto and Digital output format to Custom > DD+. Boom, Dolby Audio.
Just wanted to thank everyone in this thread for their help and insights.
I have a Roku Ultra > Vizio 1080p TV > Sonos Beam Gen 2 connected by Arc. Was only getting PCM 2.0 with any source no matter the settings I tried in the Roku base Settings menu. (I've got both the audio settings set to Auto now).
For me, pressing the Options (*) menu while playing back media brought up sound options that included Volume Leveling which was turned on (don't remember turning that on in the past but maybe I did). Turned it off and I'm now getting 5.1 from all sources that support it. Now I just have to get a better TV with eARC so I can get ATMOS working!
My Roku Premier seems to revert to PCM stereo whenever it does an update, although the audio settings in the menu are still showing that it is set for DD+. To fix it, I have to go to audio and set it to Auto, then reboot the Roku box, then go to back to audio setting, set custom and manually set it to DD+. Then I get DD+ 5.1 again. I'm not sure why this happens but when it does, I notice there was a recent update and then it outputs PCM stereo, even though the audio menu still shows DD+. Seems to be a software bug.