Reddit mentions of Wiistar 5.1 Channel AC3/DTS Audio Gear Digital Surround Sound Rush Decoder HD Player with USB Port

The problem is, you're trying to plug three 3.5mm connections into a machine that only has one 3.5mm output. Those kinds of speaker set ups are meant for machines with a soundcard more commonly found in PCs that have three dedicate outputs, one for front left and right, one for rear left and right, and one for center and LFE.

I'd recommend this optical to analog interface and this cable to connect it to your iMac with.

Ok, thank you for the insight, that is what i was worried about. I've seen these boxes:

(https://www.amazon.com/Wiistar-Channel-Digital-Surround-Decoder/dp/B0129FL3XG/ref=sr_1_1?s=electronics&ie=UTF8&qid=1491936995&sr=1-1-spons&keywords=5.1+audio+gear&psc=1)

To your knowledge would this solve my dilemma?

There are also cheap little things like this, but I don't know how good they sound.
https://www.amazon.com/dp/B0129FL3XG/ref=sspa_dk_detail_1?psc=1

I wonder if there are some higher-end versions of that.

Theoretically, there's no reason for something that simply takes a digital input signal and outputs 5.1 surround analog line level out, to be expensive. That's like, 10$ in parts and there's not a lot of "magic" involved; it's not even like a headphone amp that needs to take a line-level input and amplify it.

But, there's probably not a lot of demand for something like that, so there may not be a middle ground between $23 gadgets and home theater gear that's the size of a Panzer...