CMOS and BIOS are often used interchangeably, although CMOS refers to the actual reprogrammable chip in your motherboard which contains the BIOS, which refers to the set of instructions your computer requires to boot.
You access BIOS by pressing (usually) the DEL key during your boot up post. It should say something like "Press DEL to enter Setup" or maybe "Press F1 to enter Setup" somewhere on your screen when you boot up.
Please note that it is possible to cause irreparable harm to your motherboard/system components by editing some options in the BIOS. NEVER edit items in the BIOS that you do not understand, unless you have been instructed to do so by a skilled professional.
In this case, you want to look in the various menus for an option something like 'Use Onboard Audio' and set it to 'Disabled'. This option is likely to be found in a menu called something like 'Integrated Peripherals'.
It might also be beneficial to go into Windows Control Panel>Add/Remove Software and see if you can't uninstall your onboard audio drivers.
Also, (and this should go without saying, really) make sure you have the latest drivers for your new sound card.