It can get LOUD in the Islands, especially first thing in the morning. This video shows the call between siamangs, Olga and Rashid, but the white-handed gibbons, Chanee and Leo, can also be heard in the background. Siamangs have a large throat sac that inflates during calls and makes the resonating sound you hear. Olga and Rashid call several times a day, and their call can be heard up to 3 miles away. The call of the siamangs and gibbons will often attract large groups to their habitats, keeper

The Siamang is the largest of all gibbons and also the loudest as its musical calls can be heard through dense rainforest foliage up to two miles away. They boast a throat sac that inflates to nearly the size of its head to amplify their call. | Cincinnati Zoo & Botanical Garden