depending on the board, it should detect that you have a soundcard in a slot and automatically disable the onboard sound (PCI spec 2.1 i believe)
but if there is onboard sound, then it has to be listed under the integrated peripherals section of the BIOS
otherwise, go check the co's website, it should contain a full list of ALL recent mobos and you can download the manual for it...
Trust me I know what I'm doing... ooops, grab the cat...