First try this
Go to your start menu and right click my computer and click properties
and click device manager
after that make sure to click on the sound audio and game controllers
make sure none of them are highlighted or have any errors visible.
if there are errors then do the following
well here follow these instructions
go to the dell website or here you can go to this
Dell - Drivers and Downloads
and choose service tag I recommend that one you can usually find your service tag on the back of your computer and it would be on a tag.
once you enter that information it should redirect you to a page that shows you the type of computer you own.
after that choose audio on the file area.
make sure to download one of them that say driver because that's the base of your sound.
after you download one of those make sure to install it and then it should want you to restart your computer and your sound should be properly working