Check the return ploicy of the store you purchased the mothervoard. If your short on timeto return it consider returning or exchanging it.
If you have time to spare before returning it.
Assuming you installing Windows XP
The BIOS options for sound should be Off, Enable, Auto. Default is Auto. Change this to Enable and reboot.
Make sure you have installed the motherboard "chipset" drivers. "Intel Chipset Software Installation Utility (INF)"
If the sound chip is working the PNP OS should detect the chip and Windows Hardware wizard should ask for a driver. Cancel the Hardware wizard and boot into Windows and run the setup.exe program in the driver package.
If No Joy try clearing the BIOS CMOS memory and rebooting
If the OS still can not detect the sound chip I would consider the onboard sound defective and return the MB
If you feel lucky check the BIOS date and see if you have the latest BIOS.
If not the latest; Flash the BIOS. """BUT""" the store might refuse to let you return the MB if you tell them you Flashed the BIOS
If you can not return the MB check the Device Manager and see if you have any yellow ? marks in the System Devices.