Presently only two genera of sloths exist, the two-toed sloths (Choloepus) and the three-toed sloth (Bradypus). They are the remaining vestiges of a much great past diversity, including many of the giant forms like Megatherium that occupied niches both in the trees and on the ground. As with most fossil mammals, though, the delicate inner ear bones of extinct ground sloths have rarely been preserved, but a new paper published in the Journal of Vertebrate Paleontology attempts to analyze what the fossil sloths Glossotherium robustum and Lestodon armatus could hear based upon some well-preserved inner ear bones.
The bones used in the study were the right incus and malleus of Lestodon (the stapes was not preserved), and these were used as a proxy for the incus and malleus of Glossotherium robustum as the ear bones of the different species Glossotherium harlani are of a similar size even though the two animals did not have the same body size (Glossotherium being a little more than a third of the weight of Lestodon, which was estimated to weigh 4100 kg, or over 9,000 pounds). The estimated weight of the ear bones for both, though, was over 500 mg, putting it close to the weight of the ear bones of the living Asian elephant (which Lestodon overlaps in estimated body weight).
What does this all mean? As the bones of the inner ear become larger the ability to hear high-frequency sound is lost, signaling a bias towards picking up low-frequency sounds. In recent years it has become well-known that elephants communicate with a plethora of low-frequency rumbles and other sounds, and these sounds can travel over very long distances. Whether ground sloths communicated over long distances via low-frequency sound is more difficult to ascertain, though, and more research is required to determine how sensitive their hearing was.
Also of interest in the latest edition of the JVP (if you like sloths, that is) is an update on the giant sloth Megathericulus patagonicus, the remains of two individuals providing the first good reassessment of the species since it was described in 1904. According to the paper, it is a Middle Miocene sloth from South America, and is presently the earliest known megatheriine sloth.
Blanco, R.E., Rinderknecht, A. (2008). Estimation of Hearing Capabilities of Pleistocene Ground Sloths (Mammalia, Xenarthra) from Middle-Ear Anatomy. Journal of Vertebrate Paleontology, 28(1), 274. DOI: 10.1671/0272-4634(2008)28[274:EOHCOP]2.0.CO;2