If I want to connect my old PC 5.1 speaker set (with 3 cables, for center/bass, rear, and front) to a MacBook Air, what would I need?
An external (USB) sound card?
I saw a post that did exactly what you want. They used the headphone jack and two usb-c to headphone jack adapters to output 5.1 surround sound. I’ll keep looking for a link.
Found the link:
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Interesting!
I didn’t think that could work…
Finally received the required cables to test this… They both work fine when I test them in isolation, but only one is detected if I connect them both at the same time 
Looks like macOS detects them as the same “sound card” and cannot handle two identical sound cards?
Any workarounds for this?
(If not I might have to return them)