Our eldest will do this.. We kinda taught her to do it to curb excitement when we got home, on a couple of occasions she greeted us with her toy in her mouth, so we taught her "bear" meant that toy, when we would get home we would ask her to find "bear" - stopped her jumping up, and being all excited and stopped her barking too.
As she has gotten older and her mouth has gotten bigger she can now bark with bear in her mouth though it is muffled