Some mutants' bodies are so structured that they can generate sound outside the normal limits of human vocalization. These sounds could be of volume, pitch, or intensity beyond the norm. These sounds are usually made possible by physical mutations, but they are not always apparent.
For instance, the canon character Banshee has tough and powerful lungs, vocal chords, and throat, allowing him to generate powerful screams that can propel him through the air, destroy solid objects, and cause human beings to fall unconscious. Siryn?, on-game, has similar abilities to those of the canon Banshee, but are significantly toned down (some abilities are not even available to her at this time). Her scream is the source of her power. The XMR original character Nikki generates an even greater range of sound by vibrating her body itself, although her effects are nowhere near as powerful yet and her powers' exact mechanism has not been studied. All three of these characters have internal physical mutations, but can pass for human.
On the other hand, Ian, another XMR original character, uses a more specific form of sound generation for hypersonic echolocation. His vocal power is also based on a physical mutation, but the 'package' he got includes outward changes that mark him as a mutant.