I've tried two different Express players thinking I had a hardware issue. Been trying to get AVR to recognize a 5.1 signal. It will with Blu Ray so I know AVR is set up properly. Eventually ended up switching on YouTube Tv through the Express and BOOM instantly we're seeing 5.1 so that tells me I have setting correct in the device.
Go try it on Prime Video and it's right back to 2.1. I'm selecting movies that show they are streaming in 5.1. I'm not sure what to do to fix this. I'd really like to watch some content through these two apps in particular and have the surround sound work.
Some providers don't play a 5.1 audio track by default. Try selecting a different audio track (it's usually accessed the same way captions might be). It's not the fault of the Roku player, it's the way the provider programs their app.
I was just having this issue and changed Audio output on Roku settings from Auto to Pass through. Seems to be working but I couldn’t tell you why.
*Roku ultra