Their motivational state and health both affect color.
Bats CAN hear sounds audible to humans.
Bats make a variety of audible sounds and respond to them. Examples include communication between mothers and their young, among roost mates, and as alarm cries. Some squeaks and chitters, for example, are uttered when bats are disturbed. These sounds are often within the range of human hearing. There is a sensor in the base of the planarizing boom the tells the biped where it is in the room.