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If you are hearing crackling noises when he is breathing, then this is usually indicative of fluid in the airways, especially within the lung tissue, which can be caused by infection or inflammation.
The fact that he has lost his voice also, sounds as if the upper airway is also inflamed.
As he is also diabetic, i would recommend getting him checked at your vets asap, as diabetic animals cannot fight infections off as well as other animals, and its important to get them checked and started on antibiotics sooner rather than later, i would also be keen for his heart to be checked out by your vet as well due to the crackling noises that you are hearing when he breathes, that can occur when fluid settles in the lung tissue.
In the mean time, you could try Robitussin DM at a dose of 1tsp per 20lbs bodyweight 2-3 times daily, which should help the throat side of things, but i would get an appointment at your vets.