I've got an ancient Yamaha 5.1 receiver with no HDMI. I'm sending it audio from a Raspberry Pi4 behind my screen through a cheap USB 7.1 audio card using regular RCA connectors. The extra 2 channels are duplicates of the stereo input (using pulseaudio) and get sent to a stereo amp that goes to 4 ceiling speakers in the adjoining room. I've found that far more reliable than spdif. For example, I can download all of the Dolby test files (including their latest Atmos stuff) and I get 5.1 audio from my old receiver. Using spdif I don't.