Things like this are normally turned off and on in the BIOS.
As your computer starts up (before Windows starts to load) you should get a message saying "press Delete to enter BIOS" (message may not say EXACTLY this, and it may NOT be the delete key).
QUICKLY press the key before message goes. If you are too slow and Windows starts to load you will need to turn off the PC and try again.
Once in the BIOS look for some text which says something about internal sound card (not sure what the text will be) and select to turn it off.
Exit the BIOS MAKE SURE YOU SAVE CHANGES.
Sound should now be off.
I assume you are adding an internal sound card ?