Kacee has always been very quiet most of the time, too. Today we were driving down a street and an Amish buggy was going the other way and she barked three or four times very urgently!
I can't remember if she has seen an Amish buggy before, but she knows what horses are and I think that was what got her going more than the buggy!
I believe the secret to having a quiet pup is maximum socialization at a very early age. Of course, you also have to take into account their personality. Some are just a lot more hyper than others. Just like people.