Stereo Mix is basically a " What You Hear" mixer plug-in that will allow you to record all sounds in the current audio environment. In other words if you choose Stereo Mix as your recording line in, it will record everything including system dings, website music, whatever is playing in Windows media player, etc.
Now I haven't played with this a whole lot yet, but installing the XP drivers on a Vista system is causing the Sigmatel control panel to crash for me. I have not had a chance to test it much..
Here's to hoping Dell will actually release this for Vista users soon.
* Not holding my breath *