Hot drinks are soothing; They’re good for the voice, right?
A nice hot cup of tea is comforting, but even after you remove the caffeine, the lemon, and the honey or sugar, you're still left with a problem: the heat. The temperature of a fluid affects the tissue it comes in contact with. Heat causes blood vessels to dilate (open up), allowing the liquid portion of the blood to leak into tissues spaces, which causes swelling. Even though the liquid you drink goes nowhere near the vocal folds, they're affected because all the blood vessels in that area- the head, neck, and throat- respond to the heat of your beverage.
Cold beverages cause blood vessels to constrict, lowering the amount of fluid in the tissue, causing it to become dry. So, if you want to be in great voice, drink cool water or warm water- like warm decaffeinated tea, but try to avoid piping-hot and iced drinks.