Fire Stick no AC3 Pass-Through



I've been trying to get Plex to do AC3 pass-through but have not been successful. We have an older receiver so the TV does pass-through to it through optical. Everything else I have hooked up works fine. In fact, we even get 5.1 surround through Netflix on the Fire Stick. I have the Fire Stick set to AC3 only.

I'm not sure why this isn't working. I've thought of getting an HDMI audio splitter device that provides it's own EDID. I'd like to switch from using SageTV to Plex for our daily watching but not being able to get surround sound is a deal breaker.