Echo uses his audiokinesis to amplify his screams.
Audiokinesis (also called Voicification) is the ability to mentally manipulate sound waves or to go beyond one’s current vocal ability. This would allow one to project one’s voice or any other noise at extremely high decibels or to manipulate the volume of any desired sound, as well as what sound is projected. One with this power can even use Acoustic Location to see in utter darkness, much like a bat or submarine's use of sonar.
Some with this ability have a specific way of using it that varies depending on genetics. Some do this by warping existing sound mentally to alter it's pitch and decibel. Some generate a specific sound wave they can then manipulate to their wishes. A specialized use of this ability is to reproduce the "sonic scream" effect. This means that they can cause intense sound and even a physical blast by yelling a certain way.
In the case of Acoustic Location, an inherent weakness would be sensativity to loud noises while it was active, though they would retain superhuman hearing. In the case of the sonic scream, the user would need to have a tough larynx and esophagus to accommodate the strain.
Some alternates with this ability develop an associated mental disorder that causes them to perceive colors as sounds.