I assume you have tried going into your Hardware Manager, right clicking on your sound driver/card, chose properties and then chose disable this device.
Or restart windows and enter your BIOS and disable your on board sound via the MOBO settings.
EDIT - But why waste your time, SH3 is hungry for CPU & RAM. On Board sound eats CPU power like you wouldn't believe. Less strain on the CPU = Better SH3.
There done it for you.