How do I get my sound to work?

Since installing Windows 7, my sound randomly stops working. It's not muted. When I look at the volume in the bottom right hand corner, the green and gray bars are moving, indicating that sound should be audible. I've gone into my computer management and control panel, and everything appears to be normal. The only solution I have found so far is to restart my computer. Does anyone know how I can fix it without having to restart?