Try again enter bios then disable the onboard sound (make sure the soundcard (pci) connect the slot firmly. then check ur device manager and click twice sound, video and game controller to see the list drivers, make sure there are no yellow question mark otherwise u have to uninstall the yellow driver.(make sure the driver listed under sound,video and game controller is the same driver as the pci soundcard(not the onboard sound driver). remember before install the pci soundcard driver, please remove the old sounddriver.
as highlander says 'There only can be one' so make sure turn-off the onboard sound.