Do you have sound regardless of the red X? This happens to me from time to time on my other Windows 7 computer and since I always have sound along with the red X, I choose to ignore it, figuring it's just a glitch of some kind.
I found a discussion about this here.
On another Windows 7 forum, I found this:
I've noticed this on the odd occasion. It's as if the icon status isn't reflecting the physical status. I've found that if you toggle the mute it tends to correct itself.
If you have no sound when you see this X, then of course that's a different story.