Thanks for the suggestion about changing the video setting to composition. I opened one of the many videos with which I've had this problem (only audio plays via QT, but both video and audio play via iT).
Recounting the problem (on G5, 2x2G, 4G RAM, 10.5.1; using registered QT Pro 7.4.0 (92)):
(a) Only audio plays with some QT files.
(b) Both audio and video play when file is imported into iTunes.
(c) Problem appears to be file specific; may only occur on files downloaded via TubeSock,
(d) but appears to occur for files downloaded recently as well as those downloaded earlier.
(d) For affected files, iMovie imports only the audio, too (see example at
http://johnl.edschool.virginia.edu/QTVideoProblem.mov).
(e) Affected files have same problem when moved to another Mac using current QT (MBP 10.5.1).
Sigh...so much for using these videos in this afternoon's class.
Message was edited by: jwl3v