Titan had a puppy bark for a long time.. and to this day if prompted to bark.. he wills still start with a high pitched yip and I have to work him to give the deep bark.. not come to the door.. entirely different sound, lol
May just be his age and learning to bark. Have you talked to you vet about it? maybe something with his vocal cords?
Commander Charly Titan (GSD)-15 Jan 2010