As of Friday November 20th, 2020 5.1 audio stopped working. Have adjusted every setting and nothing. On app I use to see 5.1 and or atmos logo. Now nothing.
Factory reset your Roku device. Set it up as new.
Not sure which model Roku you have, but you need to make sure:
1) Settings/Audio/HDMI="Auto detect" (or ="Auto passthrough" for the 4800), and
2) Settings/Audio/Audio mode="Auto (DD+ etc)"
Also, if you enabled any "volume modes" (leveling, night mode) this will disable 5.1 output - make sure you disable all "volume modes" to get 5.1 back.