Kol Isha

Question

In pre-school settings, the female teachers sing with the children all the time. Is it a problem for a man who works there to be hearing this in the halls, etc.? To what extent does he have to avoid entering certain rooms in order to avoid hearing kol isha? Is there an avera for the women to sing if the men might hear them? They are very religious and would want to know if this is a real issue.