Musty odors come from mold growth. Identify moldy materials, and remove the mold by cleaning or discarding the material. Ventilate with outdoor air if outdoor relative humidity is low. Moisture can be added to the basement by ventilating a cool basement with warm outdoor air. Reduce the humidity using a dehumidifier or air conditioning. Use a fan to circulate air into restricted areas. Find and fix the source of the moisture, or the mold and musty odor will return.