1. Go into the folder Applications/Utilities and open the Audio MIDI Setup app. Check the properties for Built In Output. It might be set to 96KHz. Change it to 44KHz. Make sure the Default & System Output options re set to Built-in Output. Next, where it says Properties for:, make sure its set to Built-in Output. Last, make sure the option for Source: is Internal Speakers.
2. Reset your PRAM.
3. If you have a laptop with no sound, check your output jack on the side. Is there a red light on inside? If so, the analog/digital switch is stuck and the laptop wont have any sound. Take a wood toothpick and insert it gently in the output sound jack and press gently downwards until the red light (LED) is off. You should have sound again. If you cant get the light to go out, you will have to have it serviced. Contact Apple.
5. Try cleaning your User and System cache files with a utility thats compatible with Leopard, like Cocktail 4.0.1 or Leopard Cache Cleaner.
4. Try deleting the following files after cleaning cache files:
/System/Library/Extensions.mkext
/System/Library/Extensions.kextcache