.mp4 (and .m4v, .avi, .mov etc.) is what is known in the video world as a container. It's a bit confusing to get your head around but it basically contains the audio and video which use specific codecs.
The codecs are the important bit (and what some people confuse with the file itself). You see, the containers above can contain different codecs, such as:
H.264 for video and AAC for audio in .mp4, .m4v & .mov
MPEG-4 for video and AAC for audio in .mp4, .m4v & .mov
DivX for video and MP3 for audio in .avi
Those are just some examples.
What this means is that even though you have a .mp4 file, if iTunes or Quicktime doesn't support the codec (in the OP's case it sounds like Quicktime supports the audio codec but not video) it won't play it.
MPEG-4 is an International Standards Organization (ISO) specification that covers many aspects of multimedia presentation including compression, authoring and delivery. Although video compression and file container definition are two separate and independent entities of the MPEG-4 specification, many people incorrectly believe that the two are interchangeable. You can implement only portions of the MPEG-4 specification and remain compliant with the standard.
QT supports h.264 limited. only MPEG4 part 12 I think. No ASP and B-frame.
you can try this
<embed height="260" width="320" flashvars="file=yourvideolocation" allowscriptaccess="always" allowfullscreen="true" quality="high" bgcolor="#FFFFFF" name="ply" id="ply" style="" src="http://www.med.unc.edu/jwplayer.swf" type="application/x-shockwave-flash"> </embed>
There is a another workround if you are in fcking MS-Win:
Open Quicktime player itself, then Edit/Preferences/Quicktime Preferences/Advanced.
Under Video, uncheck "Enable Direct3D video acceleration", then select "Safe mode (GDI only)", apply/OK. (I'm not sure whether the uncheck is necessary, but it's what Apple told me!) Then reboot, open iTunes again and try to play the podcast. If it didn't work properly the first time, now it should play back properly, both in sound and video.
