Does anyone know how to get rid of a cough with mucus (vocal problem)?

I have to sing Monday, Wednesday, and Friday of this week, and I've recently developed one of those thick coughs with mucus. Whenever I cough I get this pain near my tonsils. I also have a soar throat off and on. At first I just had a soar throat on Monday. I've been taking that horrible Robetussin stuff to try to get rid of it before it actually gets bad. Does anyone know of anything else? The upper part of my range, especially my A flat above the middle C, that any choir director would love to hear from a high school tenor is completely gone. How can I get rid of this horrible cough?