Let's assume that your speakers are working (you've tested them with some other source of sound recently), your mixer and output settings for onboard sound are correct (on-board cards have configurable outputs, e.g. 2nd speakers vs. line-in etc.). If all of the above is true, then check the following.
- BIOS configuration for you on-board sound controller.
- motherboard jumpers, which are used to short the pins on audio pin header (which is used to connect to the front case panel audio inputs/outputs). Verify this with your motherboard manual.
Where did you get your sound card drivers from?
What operating system are you using?