On your keyboard press down on: Windows Key + R ...this will open a window with a textbox in. In that textbox type this: compmgmt.msc ...this will open a window called 'Computer Management'.
In the 'Computer Management' window; down the left panel select 'Device Manager'. In the bigger frame select: 'Sound, video and game controllers' and see if there's any yellow symbols with an explanation mark in it or if there's a big yellow question mark beside the controller. If there is right click it and select Update Driver; if you have a CD for your computer then now would be a good time to insert it and then follow the onscreen prompts. Otherwise look up what Sound Card you have, and download the drivers from the manufacturers website. Then depending on the software that comes with the Driver Package (you've just downloaded) you can either install them by running a wizard from the driver package or follow the above steps and install the drivers that way.
By installing the drivers the system will know what the device is and will resolve the problem.
That is the best support I can offer due to your minimal post. If you would like to provide more information (i.e. Operating System, the Sound Card manufacturer and model and how the system notifies you of not recognising a Sound Card) then I may be able to provide more comprehensive support.
