Hi. You probably need to load the sound drivers for your computer. Here is the Link to Dell's driver for your model:
http://support.dell.com/support/downloads/driverslist.aspx?os=WNT5&osl=EN&catid=3&impid=-1&servicetag=&SystemID=DIM_PNT_PII_XPS_R___&hidos=WW1&hidlang=en
Download them to your desktop and then press "Start" "Run" and "Browse" to your desktop and the file you downloaded. then click "OK" and follow the directions on your screen. Hope this helps
Bruce
June 2008