I don't know how else to describe it. When Mia is happy (especially when she has a toy she's chewing on) she chirps. It sounds almost like a bird. Really high pitched and kind of weird. I wish I had a video camera.
Izzie has one bark for alerting and general pug-assery, and then she has her chirp bark. Usually the chirp comes out when i have her Cuz ball, she's in love with it so if i don't throw it she freaks out and chirps.
She also has a sound that is more of a "Woo Woo" than a bark, she has her wookie sound too, and her other general grunts and snorts too