Welcome to the forums.
You need to install the audio drivers. It sounds like you have a X-Fi sound card in the system, because usually Windows will install the onboard sound during install.
If you do have the sound card, will need to download and install the Sound Blaster X-Fi Xtreme Audio Driver from the 630i Download Page. Make sure you choose the correct version of Windows that you are running.http://supportapj.dell.com/support/downloads/driverslist.aspx?os=WV64&catid=-1&dateid=-1&impid=-1&osl=EN&typeid=-1&formatid=-1&servicetag=&SystemID=XPS_630I&hidos=WW1&hidlang=en&TabIndex=
The driver will be the first download under the audio section.
If you do NOT have the SoundBlaster X-Fi, then you will need to download the 2nd Driver under the audio section, for the onboard Realtek Sound.
Also, it is good to put your system specs in your sig, so we can help you troubleshoot a bit easier.
Hope that helps.