I had a cat do that. She sounded like someone who had screamed too much at a football game the day before. :) It took a few days- 3-4 and she got it back.
Is he running a fever? Has he been neutered? I had a Siamese or two who actually meowed like that a lot....it was just their normal way of meowing. How old is your kitty? Maybe his voice is changing because he is going through puberty. I'm sure he will be fine, he doesn't sound sick to me.