what is the sales tax on a $9.99 cd?

Whatever your state and city charge. My crystal ball being out of order, I have no way of knowing where you live so I can't give you specifics.

The iTunes Store does not sell CDs, so not clear why you are posting here.

If by any chance you are curious about the sales tax on albums or other items downloaded from the iTunes Store, it is the same as you would pay on the same amount at your local bricks-and-mortar store. Where I live, that is 7%, which comes to 70 cents on a $9.99 item.