I've had mine for a week and last night I heard them bark for the first time. My husband has heard it before but I always slept right through it.
But last night I heard one of them barking and I rushed into the living room to see. One of them was clinging to the cage with the face pushed right up against the bars barking. I couldn't tell if it was Lessa or the male whom I have yet to name because I didn't want to turn on the light. I was afraid the light would make them stop. One thing I did notice though is that the other one would stop for a second then go crazy running around the cage. I've read of other's gliders standing still to listen, but mine just flipped out. The babies were in the pouch, so I couldn't see their reaction. I have no other pets, there was nothing weird happening. So I guess they were just being weird. =]