1) Are you sure the sound card drivers are installed properly?
2) Make sure all the cables are plugged in to the correct audio jacks and the speakers turned on/up.
3) Make sure mute isn't turned on.
Check out this site. It will be able to give you some good information and great troubleshooting tips that should be able to help you solve your audio troubles!
It's hard to figure out why you and your computer aren't on speaking terms without knowing what computer, card, speakers you have, and what you've done to it lately.