She may have an injury from swallowing something big and scratchy, or it could be any of the above.
With Carmella, after a few days of no improvement, I took her to the vet to be sure there was nothing in her throat, including gapeworms.
She looked fine, lungs clear, all that.
So I took to home remedies. I had some colloidal silver liquid, so I put a teaspoon of that in a couple of gallons of water overnight. It could be coincidence, but the next day she had a little squeek. In about four days she was quacking again. There is a caution not to use c.s. for very long, I would not use it more than a few days.
And the disclaimer here is I am not a vet, just a duck person, I cannot say the colloidal silver did anything. So do some research and see if you want to try it. I use it on myself from time to time.
Another thing to try would be apple cider vinegar in the water. I use that as a tonic from time to time. A weak solution is what I would try, since if she's got scratches inside her throat I would not want to cause it to burn.
Some very moist rolled oats might also soothe the throat. If she eats dry pellets, I would moisten them for a week to prevent scratching. Be sure not to let the food get moldy, though. You could combine water with just a little (teaspoon) of apple cider vinegar and add that to their food.
Those are my thoughts. It was really worrisome, and I am glad her quack came back.