It's very easy to do via the PCs Bios.

There are different ways to access the Bios settings depending on which motherboard / Bios is installed on your system. When your PC starts, the first screen you see is black and white and displays your IDE hardware and your Bios information. Some PCs will also display the Bios access key (Delete, F2, F8, F12, etc.) at this point but if not, note down the Bios information and locate your Bios / PC on this list to find out which keyboard key(s) you will need to access the Bios at start-up.

Restart your PC and when you see the black and white screen displaying your IDE hardware, press the required keyboard key(s) and your Bios will appear. Look for a setting related to your hardware (e.g. Integrated Peripherals) where you will find your on-board audio settings with an Enable / Disable option. Once the setting has been changed don't forget to save your settings before you exit the Bios.