I have had the same issue with various m4v files that have been run through metax for no discernable reason or logic. They are tagged right the first time, but any attempt to alter the tags down the road with metax, lostify, or itunes, and the file becomes corrupted with either no sound, no picture, or both. Dynaflash - if you're reading this, you might know the answer. What are tags most of us don't see in an m4v with relation to resolution, audio, and video properties? What I mean is, are there tags in the video that might have been mistakenly modified that we don't usually see in metax or the 'get info' tab in itunes that are needed to be read by quicktime?