I had the same issue and after doing some troubleshooting with Google and researching on Netflix and Prime Video, I basically came to the conclusion that streaming services don’t support 5.1 through your browser.
Awesome! I thought browsers just couldn’t output 5.1 but, hey, this worked, thanks!
For Netflix I use the Windows 10 store app now, which bypasses the browser so I can get full 5.1. For Prime Video, there is no app so you’re stuffed unless you have another device. For me, I use the PS3 for that, which works fine, depending on whether it needs (another) update.