I'm afraid we need a bit more info.  How are you getting to the box marked "Entertainment?"  I can't find that in my control panel in the Sounds, Speech, and Audio section.  The way mine works is if I open the Control Panel, Sounds, Speech, and Audio, Sounds and Audio Devices section and click on the tab marked Audio, I get a box that allows me to select the default playback device.  If you click on the arrow at the right of the box marked "Default Device" you should get a list of available devices.  When you do that do you get any choices, or is the box empty?

 

     Ok, I think I get it.  How did you find the driver for the Sigmatel audio?

    

     While I await your answer, since you don't know if your computer was ever set up for audio I think we should start at the beginning.  Restart your computer, and when the first screen display appears (usually a Dell logo), press F2 to enter bios setup.  Use your arrow keys to navigate down the menu until you reach Onboard Devices.  Select it and find the entry for onboard audio.  Make certain that this has been turned on or enabled.

     

     If this has been enabled, next step is to go to the Control Panel, go to Sounds, Speech and Audio, and once there go to Sounds and Audio Devices.  Click on the Audio tab and make certain that the playback device has been set to Sigmatel High Definition Audio.  If you can't do that let me know and we will explore other possibilities.