I have recently bought a MacBook Pro 13" and want to use my old speakers to play music through, but the system can't recognise the speakers are there? They are connected through the headphone jack, to be played through the AUX setting on the speakers.
First, plug the speakers into something else to make sure they work as-is.
Next, check the headphone jack to see if there is a red light emanating from it. I have never personally experienced it, but I have seen reports of MacBooks being tricked into "thinking" that it should be in optical output mode and getting stuck that way. If so, sometimes removing and replacing the speaker cable a few times may help. See https://discussions.apple.com/message/22315967 for example.