I think your Sony receiver is automatically doing a "Dolby Pro Logic" de-coding on 2-channel audio from that source. You should look into your receiver documentation. There should be a way to change the sound mode from "Dolby" to "Stereo". This could be a permanent default or a one-time change whenever you need.
When I'm using Netflix, the sound seems to be okay, as far as the proper Dolby or whatever being used. But if I use another app on the Fire TV Stick, one that pretty much just sticks to basic two-speaker audio - like Tubi, Kanopy or Hoopla - the sound on the receiver is always "surround" and using the 5.1 setup. Shouldn't this default to just basic 2.0 sound since that's the way these apps are sourcing their material and many of these movies are older, pre-Dolby stereo mixes anyway? https://omegle.onl/
issue got solved!!