Not certain I follow. Is your Blu Ray player also your amplifier, i.e. it's a "home theater system"? If so, the only way you'll get surround sound is if the BD player has an HDMI input you can connect the Roku to. If your TV doesn't have optical out, there's no way to get the audio back to the amplifier. Perhaps I'm not completely understanding your configuration. How is the TV audio getting back to the amp?
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.