We know what an echo is. What we need is more details about your exact problem. Is it a single video, several videos from a single source, or from different sources? And what are the sources? What format(s) are they in and what codecs do they use? Does the problem happen with every multimedia player you try them on, or just one? What have you tried so far to fix the problem? And so on.
If the echo is in the original audio itself, then the only likely option is to separate the audio and video streams with a media editor of some kind, such as ffmpeg or avidemux, run the audio through a sound editing program like audacity or sox to remove the echo (if possible), then remultiplex the fixed stream back in with the video.
Last edited by David the H.; 03-27-2011 at 04:15 AM.
Reason: small addition