Have you tried to just install the other soundcard and see if it can be used or not. In some instances the installation of another sound card will auto disable the built in one.

If not the place to disable it is from within the BIOS itself and not through jumpers. Open the BIOS (set-up) on your computer and look for the peripherals
heading and under that you should see audio setting and change it to be disabled. Then F10 to save and exit.