Originally Posted by tredegar
A very common problem is that some sound sources are muted or are set to a very low volume level.
Open up your sound mixer, and make sure sources aren't muted, or turned right down.
What version of Ubuntu are you using? Ubuntu/Kubuntu/Xubuntu, and version 6.06/6.10/7.04. All of those version numbers are still regularly used/downloaded by many, despite the fact they are a bit dated(6.06, 6.10).
If you have the "horn" in your taskbar, right click it, and choose "Open Volume Control", and make sure everything is all the way up(especially Master, and PCM).
If that checks out, click File/Change Device, and see that its set properly.