Acoustic communication plays an important role in the ongoing success and survival of mammals in a rapidly changing world. Anthropogenic activity is increasing and spreading at a steady rate and it is important to understand how this activity has the potential to impact mammals’ ability to communicate. The first step is to have a strong knowledge of the acoustic communication abilities of as many species of mammals as possible. We compared the acoustic signal design (vocalisation) and reception (hearing) thresholds of over 190 species of mammals to examine which life history traits are responsible for driving their diversity. We found that the same two drivers, environment and body size, were responsible for the majority of variation in mammals’ acoustic communication thresholds. As had been found in early smaller studies on terrestrial mammals, we found that larger mammals produce and receive lower frequencies than smaller mammals. However we also found the surprising result that aquatic species produce and receive signals at higher frequencies than their terrestrial counterparts. Aquatic species had previously been largely underrepresented in previous comparative studies, and including them uncovered some interesting and unexpected results. While we expect a large mammal to have a deep booming voice, we found that large aquatic mammals are capable of producing high frequency clicks and whistles as high as a mouse. Our results provide a bigger picture for mammal acoustic communication and can hopefully be useful in easily determining which species may be affected by increasing levels of anthropogenic change in the environment.