Does anyone know what it means when your dog suddenly stops barking and is howling?

My dog is 13 yrs old, he has cancer but he's a trooper and is doing ok. He eats fine and still runs even though his leg is ailing him. His bark was a very strong bark, he is a black lab and german shepard mix, but the other day instead of barking, he howls now. Any ideas of what's going on with him?