When I think of a sloth, I think about it being slow, lethargic and silent. Why I associate silence with a sloth is probably because I envision it creeping slowly through the tree tops and then stopping to take a long rest. An animal that's so slow brings to mind silence, kind of like a turtle. Bottom line: I just don't think of a sloth as a noisy being.
Well, it turns out that they do make sounds. The babies make bleating type sounds if they become separated from their parent. (Hey, it's a jungle out there and those little guys have to have a way to let mom know where they are!) Once I think about it, it would be pretty sad if the babies didn't have a way to alert their parent. So, being able to make a sound makes sense.
Babies also make squeaking sounds in certain situations. I found a video on the Internet of some baby sloths in captivity. The caretakers were bathing them and the babies were not pleased. They were making high-pitched squeaks, which I assume were a sign of their distress.
Adults, at least the three-toed sloth variety, can actually make long, high-pitched sounds that echo through the trees. Why they do this, I don't know. Perhaps its how they communicate with their sloth community? Or maybe they just like to hear themselves cry out.
Sloths may make other sounds, too. Maybe humans just haven't witnessed them. No matter whether sloths have a wide and varied bank of sounds, they do have a means of communication and they use it.