The vocal folds are in the syrinx which is located at the of the trachea and then is divided into left and right bronchus. Once a birds voice changes it may be a sign that that area is inflamed or infected possibly even a fungal infection which is a fairly common cause of birds voice is to change.
There can also be something stuck in their trachea, a piece of seed a splinter from a toy or even a piece of fungus. You need to be careful because any of these could obstruct airflow into the respiratory tract.
You also need to rule out inhalation of any possible toxins such as cigarette smoke, gasoline fuel, hairspray, cleaning products so forth.