Can you tell us what you see when you to into the device manager and click on the plus next to "Sound and Video Devices".
You may also want to look at this part of your motherboard
The small square chip in the picture is your sound chip. Yes, it's really that small making all that sound. Look at the capacitors in that area and also at the solder points on the edge of the chip. Something may have come in contact with this chip or one of the capacitors and knocked one loose or broken the chip.