I would check the System Profiler under System Tools and make sure your audio adapter is listed under the summary tab. If it is, then you can search for issues with that particular adapter.
Press Ctrl+Alt+T (opens terminal), type "alsamixer".
Look that there is no mute and volume is ...to the MAX! :)
<sigh> It is impossible to help in a vacuum. You've given us absolutely nothing to go by. See this sticky.