Having signed up for the 99c/month Hulu deal I noticed that for some devices Hulu offers 5.1 audio (DD+) but on selected devices. The list includes

• Chromecast*
• Fire TV (3rd generation and Cube) and Fire TV Stick 4K
• LG TV (WebOS 3.0 and above)
• Nintendo Switch
• PlayStation 4 (all models)
• Xbox One (all models)

I was watching on a Fire TV stick regular and didn't get it but when I moved to my other HT system with the Fire TV Stick 4K the DD+ light came on the AVR. Nice.

I wonder why they are so selective.  Netflix offers 5.1 audio on most of their devices from what I can tell.