If sob is caused by mucus can. Cough syrup expectorant solve the prob?

Possibly. Anyone with thickened secretions can be aided by a mucus thinner like Mucinex (guaifenesin) and drinking plenty of water. If you have heart failure however you shouldn't overload your body with water.
IT WILL HELP. But i would not go as far as saying it will solve the problem. You need to treat the cause of mucus production.