You have to go to you manufacturer site, under Drivers and support, enter your pc model or select it from a list and download the sound card driver and install it.
If it's a "home made" (as I like to say ) computer U need to find what sound card U have and then search for the drivers. Look in system info, or right click my computer>Hardware device manager, you should have an ! mark. Try to update. Or U can look in Start>Run>dxdiag under sound.