You need to make sure that HD sound is selected in the board's BIOS. If AC97 is selected, the HD driver will not find the sound chip.
To enter BIOS, when you boot up the PC, start tapping the DEL key. You will get a blue text screen with lettering. Should be Integrated Peripherals, a listing for what type of sound.
Also, if this is a home built PC, the front panel wiring must match what the BIOS is set at. Or if the front panel does not have HD sound wiring, it has to be left unconnected. If the PC was bought pre-made, they should have done it right at the store.
Of course, if this PC was store bought, they should have set the BIOS already for HD audio.
1)GA990XA-UD3, Phenom II 970 ,8Gb DDR3 1866, EVGA GTX560Ti,500Gb SATA II , LG DVD, Win 7 Ultimate 64bit
2)AX78, Phenom 9600, 9800GTX, XP Pro SP3