Is this the result of reinstalling an operating system?
After the installation finished there should be another CD with a whole set of Drivers on it, specifically arranged to be compatible with various ancillary operations in your computer – sound and screen resolution are two of those ancillary operations.
Install the Drivers on the CD or go to the manufacturers website for your motherboard. Download the Chipset Drivers from there.
ALL OF THEM – there should be between 5 to 8 of them depending on what motherboard you’ve got. 🙂