What causes distortion in the audio?

The primary cause of distortion in audio is clipping. Clipping occurs when the audio signal exceeds the maximum level that a system can handle, leading to a waveform that gets "clipped" off at the peaks and valleys. This results in a harsh, distorted sound, as the audio is no longer a smooth waveform but has flat peaks where the original signal has been truncated.

Peaking refers to a momentary rise in audio levels that could lead to clipping, but it is not in itself the cause of distortion. Noise generally refers to unwanted background sounds that can interfere with the quality of audio but doesn't inherently cause distortion. Echo is a delay effect that creates a repetition of sound, which does not lead to signal distortion. Therefore, clipping is the clear choice for the cause of audible distortion in audio signals.