I expierienced the same, building a new PC with older hardware (AC97 onboard sound). I found out that two jumpers were missing on the mainboard since no AC97 front connector from the case was connected. With the jumpers missing the mainboard "thinks" the sound is routed to the headphone jack, so no sound is routed to the back panel jack.
"In the AC'97 design, the audio output is sent to the jack. If the headphones are not plugged in, the jack directs the audio to the return pins that are connected to the speakers. When a headphone is plugged into the front panel jack, the audio signal goes to the headphones; the return pins are disconnected, so no audio signal goes to the speakers. Many motherboards with AC'97 audio need two jumpers to short the pins in case no front panel audio module is connected."
...old thread but I believe this may help a few others dealing with older hardware.