I'm not sure how much further I could help because BIOS options are different based on motherboard model. There is usually a hardware section where you can adjust startup options, voltages, frequencies, and other such items. All I can really tell you is have a look through all your options and look for the Onboard soundcard and your Soundblaster, then just disable the onboard.
As for what relates to the soundcard, the items should be very simplistic. BIOS stands for Basic Input Output System. And what it is designed to do is recognize hardware and tell the operating system what's there, how to run it, and where it is. So in terms of what you ought to see, it should be barebones. Part names, basic overclocking options, startup order, the voltages and frequencies everything runs at, and some other basic options depending on the parts you have such as Fan speed and the like.
Sometimes the soundcard is not listed in the BIOS. For me, my onboard soundcard was not listed in my BIOS. I had to use the Device Manager.
So have a look through it, and if you don't find it in there, it's likely that it isn't in the BIOS and the Device manager is the only way to go.
I'd also look for and uninstall the onboard soundcard drivers if you haven't already. They should be listed in your Uninstall Programs list.