Clipping occurs when the leading edge of a sound is lost. The most common cause of clipping the the latency between detecting the sound and activating transmission, especially in a circuit switching-based implementation. Packetized voice usually eliminates clipping by sampling and packetizing all samples, then suppressing those that are "empty."
This demonstration has two samples: one with the entire sample as shown below and one with the first 50 milliseconds clipped off.
Source: Screen capture from "CoolEdit" - http://www.syntrillium.com/
Sample without clipping
Sample with clipping
Return to Menu of Sound Samples