Well, everyone coughs now and then. The less worried you are about coughing, the less likely you are to feel the urge. If you have a cold, take some cough medicine in advance and bring unwrapped lozenges with you. Try to cough during a loud passage of music. If it is impossible to control your cough, it is perfectly acceptable to quietly exit the auditorium. The ushers will tell you when you will be able to re-enter the hall.