First things first; update your operating system. One of the first steps to troubleshooting any problem is making sure you have the latest software running on your machine. If you're running Mac OS X Snow Leopard, you should have version 10.6.3. If you do not, use Software Update in the ? menu or download the combo update directly from Apple, here.
If your OS version is correct, the next step is to check the sound preferences in System Preferences > Sound. Click on the Output button at the top of the window and check the device list. Select, Internal Speakers, and test your sound. Slide the output volume to a louder volume and be sure the Mute button is not checked.
If the Internal Speakers option is not available (for Macs that are not Mac Pros), you may see the option as Digital Out. If this is the case, check to be sure all audio devices have been disconnected from your computer. Next, check the audio line-out port for a red light. If the light is present, you may need to take your Mac in for service or call AppleCare. If you have completed the above steps and still cannot get audio from your internal speakers, it may signal an issue that needs a service technician to fix. Make an appointment at your nearest Apple Store Genius Bar, an authorized AppleCare technician, or call AppleCare.