The hyoid is a small bone that anchors the tongue and larynx in most mammals. In howler monkeys it is greatly enlarged and hollowed into a rigid shell that sits beneath the jaw, forming a resonating cavity through which air from an enlarged larynx is driven. The result carries for kilometres through closed forest, which is where the chorus does its work: troops locate one another and space themselves without meeting.
Across howler species the size of that bone varies, and comparative work has found a trade-off. Species with larger hyoids tend to have smaller testes, consistent with investment either in loud advertisement or in sperm competition rather than both. The pattern also tracks social structure, from single-male groups to multi-male troops.
The common claim that howlers are the loudest animal on Earth is overstated. Several whales, and some insects relative to body size, exceed them; the reasonable claim is that howlers are among the loudest terrestrial animals.