A sound card is a part of your PC which handles playing any sounds. Music/sound effects - everything.
It is called a Sound Card because in many PCs this is a separate circuit board (card) which plugs into your PC. On some PCs the "Sound Card" is part of the PC's main Mother Board. This type is called an Integrated Sound Card.
Your PC speakers (assuming your have external speakers) plug into the Sound Card.
If your Sound Card is broken you should check which type of Sound Cards will fit into your PC's Mother Board and purchase a new one. The good news is that a basic sound card is not very expensive. Better quality ones for demanding PC Games or music mixing applications are much more expensive.
Craig
August 2005