You've got a bad connection somewhere. Either one of the cables isn't fully plugged in, has some sort of defect in it, or one of the connection jacks is loose and not making proper contact for signal relay. Pray that it's the cable, because if a jack has been damaged, you're looking at getting a replacement for whichever part has the damaged jack. That is unless you, or someone you know, is an old hand at fixing electronics like that.

It should be easy to test the cable. Just take it to the nearest Radio Shack, or electronics parts store, and get a properly gendered cable of the same jack size.

Also, unless the speakers have some system of upmixing audio signals, if you have a stereo source, you're only going to get audio from the front two speakers. That is how it should be. Not sure why this seems to throw so many people... I've probably just had an understanding of this for so long I don't remember what it was like before any longer.