The first place I would look would be in device manager in windows. Look for any yellow exclamation marks. If you find any post back with the details.
When you formatted the hard drive did you reset the bios? This should be the only way that the on-board sound could be re-enabled. If you did reset the bios then you are going to have to go in there and disable the on-board sound chip.
All this presumes you have a sound chip on the motherboard and a separate PCI sound card installed.