The best thing to do here is that you can open device manager in your system and firstly uninstall the sound driver, then restart the system and check whether it gets fixed or not.. If not then please uninstall the sound driver and then go to the link http://www.dell.com/support/drivers/us/en/19/DriversHome/NeedProductSelection
and select the model number for your system and install the latest sound driver.
I trust this will fix the problem
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