Problems with HP JBL Platinum Computer Speakers

I have JBL platinum computer speakers from an old Presario I had. I ear the sound perfectly from the speakers to ear anykind of sound from PC (music, video, etc) BUT if I plug headphones directly to the speakers appropriate plug NO sound comes out from the headphones... if I plug the headphones directly to the PC jacket (rtaking off the one from the speakers) it WORKS. I have checked software (realtek) , tested changing set ups, bla bla, ans the SAME result. Hardware problem (sound card? what?) My PC is new and works properly as I said. Thanks JR