Hi,
Do you remember making any recent changes to the computer prior to this issue?
I would suggest you to follow the steps mentioned below:
Method 1:
Open audio troubleshooter and check if the issue occurs follow the steps in the link below:
http://windows.microsoft.com/en-us/windows7/Open-the-Playing-Audio-troubleshooter
Method 2:
Verify if the default sound device is set correctly. Follow these steps:
a. Click the Start button, click Control Panel, click Hardware and Sound, and then click Sound.
b. Click the Playback tab. Make sure the correct default audio device is selected. (For most users, the default audio device is Speakers.)
c. If your default device isn't set to the correct audio device, on the Playback tab, select the correct device, click Properties, click the Advanced tab, and make sure that Default Format is set to the value that you expect. When finished, click OK.
d. On the Playback tab, select the playback device, click Configure, and then make sure the configurations are set correctly.
If the sound settings are set correctly, and your speakers are already set as the default device, go to "Verify that an audio driver is installed" now.
e. Select the audio device that you want as the default audio device, and then click Set Default.
f. Click OK to save your settings.
For your reference follow the steps in the following link:
http://windows.microsoft.com/en-US/windows7/Tips-for-fixing-common-sound-problems
Method 3:
If the above step fails follow the steps from the link:
http://windows.microsoft.com/en-us/windows/help/no-sound-in-windows
You may also download and install the latest audio driver from dell website:
http://www.dell.com/support/drivers/us/en/19/DriversHome/NeedProductSelection
http://support.dell.com/support/topics/global.aspx/support/kcs/document?docid=266495#Driver-Update-3