how to do this?

I just bought a pair of x 530 logitech speakers. As i said in another post, not all of the speakers are working, depending on which colored connector i put into the iMac. Is there a way i can get all of the speakers to work on an imac? Can i use a headphone splitter? I dont really want to buy anything expensive like an external sound card, because i really dont listen to much music.