On my desktop's front panel I plugged headphones (microjack) and in the back there's an amplifier. I have to move the cables so there are no hisses and other sounds. Often only one loudspeaker is working, or sound is not working altogether. Is this the fault of the sound card?
Try to plug only to the back jack and take off the front panel. It sounds more like there is a problem with the jack itself than with the audio card. You should also plug the headphones to another device - maybe they are not working properly because of a cable, etc.