Dual sound cards if not work mostly cause the system conflict, here are some suggestions you should try :
1. Plug out the PCI sound card before install onboard one and make it work first.
2. Change onboard sound IRQ and Address like IRQ 11 and 240.
3. Make sure your on-board work well with new setting then plug the PCI sound back into machine and start the PCI sound card installation.
4. If still have problem try to move your sound card to other PCI slot or force
the PCI slot to use IRQ it can be set in your bios setup.
Also not all sound cards can working when dual but mostly cause the conflict so try more other setting may get it work.