There are a number of problems that can arise with hardware on a computer. Conflicts among a hardware's driver and other programs on the computer might cause a sound card to function incorrectly or not function at all. If your computer doesn't recognize a new sound card, it can't access it at all. Luckily, if there's some kind of problem like this with your sound devices, it's generally pretty easy to determine.
Open the "Control Panel" located in the "Start" menu.
Click on "System," and open "Device Manager." On some versions of Windows, the "System" menu is called "System and Security."
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Click on the arrow next to "Sound, Video, and Game Controllers." This has a list of sound and video devices on your computer, including your sound card.
Select the device you want to check, and right-click on it. Select "Properties." This brings up another window, which says, "This device is working properly" if there are no issues with the selected device, or, "This device is not working properly" if there's some sort of problem with it.