My sound isn't working right?

When I try to listen to music on youtube or itunes or really anything, it doesn't work. But, when I went to the Realtek HD Audio Manager, it said that something was plugged into the audio jack. Then I clicked on the little tester button thing and it worked. I tried to listen to music again and no sound. Then I went to control panel>sounds and audio devices, and it said nothing was plugged in. What do I do? I have a Dell computer if that helps and the speakers say Platinum JBL Series.