It has an spdif (optical) out, eARC (hdmi audio send) and a stereo line out via 3.5 mm jack.
So it the only analogue outs is has are stereo via the jack, you could connect that to dual RCA/phono on your receiver using a suitable conversion cable. If your receiver has an optical input you could use that as well although I have yet to get any 5.1 surround mode working myself doing this.
I have ended up using a hdmi audio extractor widget off amazon, which connects between my firestick and the TV) to get 5.1 (via optical) into my old Marantz receiver.
No spdif input so 5.1 is a no go. Mind you I never got the spdif on the TV to pass 5.1 to my receiver anyway, best I got was pro logic.
Coming to the conclusion that the audio on the C1 is just best completely avoided i.e. extracting the audio by some means before it reaches the TV (which mean using an external video source of course, a fire TV in my case).