if you're on XP, go into control panel, then into system and select the device manager tab - you should get a list of all installed devices (hardware) on the PC, and will hopefully find an exclamation mark next to the sound devices, where the drivers may not be installed properly - then try the 'update driver' feature to reinstall the drivers for the sound card - i've just had the same happen on mine (new hd, had cd's for all of the devices except the sound so had to use xp drivers to get that going)
i'm not great at this stuff lol but look at the device manager to see if you have exclamation marks anywhere in the device list