[mothered]

do you think they would not do my refund if I decided to keep one of the cables? xd

Personally, I'd leave It as Is but If you'd like to go ahead with It, here's what I suggest.

Given they've accepted that the earbuds were missing, there's every possibility that the cable was also not Included so as far as you're concerned, you didn't notice that, hence only claimed the earbuds.

If you stick with that story, there's every chance that It will work In your favor. If they refuse the refund due to the cable, simply tell them that you've found It (It slipped between the couch cushions when you unboxed It) and return It thereafter.