If the 4K Fire Stick outputs a lossy Dolby Digital+ (not Dolby MAT) audio stream, it might be possible. Personally I don't have an Atmos AVR or soundbar to check...Another question for the Oppo experts:
If I plug an Amazon 4K Fire Stick in the back USB port of my BDP-103, will I be able to stream Atmos mixes from Amazon Music through my system?