Since winter mom has had dry cough to point hard to breath and loses voice- chest xray negative- breathing treatments in er work- had now for 3 months, could something serious be wrong?

Consider a few..: I see this frequently in the rough winters where i practice. Many older people become more sensitive to the cold temperatures, causing bronchospasm in the lungs and therefore coughing when exposed to the cold. Also, the extreme dryness of the air can contribute to this and cause more coughing. Try a cool air humidifier in the rooms she is inhabiting. These are a couple of possibilities to consider.