Need this cable! Help find name & source for round 9-pin on back of computer speakers

[tuqqer]

I inherited a set of computer speakers (2 small and one subwoofer). It's all there, except this cable that connects from the back of the subwoofer to what I think would be the audio out port on the Mac.

I'm having troubles figuring out the name of this kind of cable, and where I would buy it.

Is anyone good at figuring out what this is?