I had this issue. My desktop's sound was working, and I could hear videos I played on it. But not iTunes. And the volume measure on iTunes was not down and "off" but right in the middle. So why no sound?
There is the little "display" symbol in iTunes. It's important to click on the one in iTunes, as I kept clicking on the one on the main menu at the top right corner of my computer and that wasn't helpful.
When I finally clicked on the one in iTunes it let me know that the sound was on "single" and supposed to be coming through exterior speakers only. These, clearly, weren't on as I was getting nothing out of them. I changed over to "multiple," and was offered choices of where the sound should issue. One was "computer." I and ticked that off and set the volume. And that's how I got my sound back on iTunes through my computer.