Did you check the Realtek config app and/or wizard. (i think it's called HDRealtek Audio or something).
It seems the driver doesn't assume by default that the jacks are what common sense dictates they should be. I had to run the wizard and tell the stupid thing that it was speakers that I had plugged into the green speaker jacks before it would work.

Also, there was momentary confusion on my part as to whether "Front Speakers" referred to front speaker jack or front speakers in a surround system.

That said, your symptoms do sound like bad jack, cable, or something as mentioned above, but check the config if you haven't because the default settings will probably be wrong.