I find any drink with ginger in it gives relief from coughs and wheeziness. I recently got through a singing gig when recovering from a cold, with the assistance of a thermos containing Rochester ginger cordial, honey and hot water. The heat of the ginger seems to soothe the throat down immediately, even though I can't explain it in terms of physiology.

Alcohol can help relax you, but as has been implied above, the best way to get it to enhance your performance is to make sure your audience have plenty to drink!

Marje