On-board sound can't be taken out... it's soldered to the motherboard.
Look under titles such as "Advanced Chipset features" "Chipset Features", "Integrated Peripherals" or "On-board Devices" and look for something along any of those lines.
Typically what you need to look for inside those menus are somethign related to sound or audio (those words should be in the name). These may be in sub-menus from what is listed above.
If this doesn't work, you could always disable your on board sound in device manager so windows doesn't use it then install your new card. But, it's always better to disable it in BIOS if you can.