Q: record line-in audio on 27-inch iMac with Retina 5K display?

From what I can tell that model doesn't have built in analogue audio input, so you would probably need some sort of third-party USB audio interface. You might be able to record in mono with a suitable adaptor since the port is apparently compatible with Apple's earbuds for example.

tt2

yeah with a splitter for the headset which has a male 3.5mm minijack apple version with the 3 rings in one end and a normal female 3.5mm minijack in the other for audio out and a mono 3.5mm minijack female in the other should work

I've had this little gadget for quite some time - works great:

https://griffintechnology.com/us/imic