Google search for "7805 voltage regulator". Keep in mind that this approach wastes a lot of your valuable battery power as heat. Not a problem for an evening show, but on weekend festivals it adds up.
Many people use one of those little black boxes which takes 4xAA alkalines and plugs into the iRiver, and that will keep the iRiver running at an all day festival. $10-20'ish on ebay. Key word is "battery extender".