How can I identify specifications for this possible passive speaker with subwoofer? And what are each wirings on that cable for?
It's more or less impossible to find out what the specs are unless they're printed on a part or you can find original information from the manufacturer.

You can make educated guesses, though; the use of red and white probably matches the standard RCA red for right, white for left audio, and the black is probably ground, and the two black wires are therefore probably ground for the channels that they are next to. Probing it with a multimeter in conductivity mode would probably show low resistance when you have a matched pair. You could then apply a signal from a headphone output (these are nice and low power) and see if you hear anything.

On the other hand, if you take it apart and find a circuit board in there, it's probably digital.