Many of the YouTube 'Flash videos', despite their .flv suffix, are mpeg4 files in a Flash container.
If you are having problems playing MPEG4 video in Quicktime 7.4 (and Apple are aware that some users have had such problems) you should carefully check in
/Library/Quicktime
and /Library/Internet Plug-ins
and Home/Library/Quicktime
to check whether you have any third party plug-ins that are not completely up to date.
Current versions of Flip4Mac and DiVX and Perian do NOT cause failure alone or in combination. However the one SPECIFIC extension that ALWAYS causes the failure alone or in groups is
3ivxMediaImp. All other 3ivx extensions DO work. Remove only that ONE SPECIFIC extension.
Please try this and see if it fixes your problem with QT Player, the QT Firefox Plugin, and iTunes not playing MPEG4 files (including mp4 files).
Older 3ivx and DivX components are often incompatible with new QuickTime versions and are, in many cases, obviated by component packs like Perian.
This is not the only explanation for issues with MPEG4 file playback in QuickTime 7.4 and later, however. The issue with videos from democracynow.org and others, for instance, is the result of that content not complying with MPEG4 standards (namely that they use bogus track dimensions). QuickTime had previously ignored invalid track dimensions but this was changed in order to support properly authored anamorphic content.
The recent update to QT 7.4.1 has fixed a number of issues but some problems are unavoidable when working with non-standards compliant content. The standards are important for many reasons including keeping hackers from manipulating improperly coded web pages.
However, there is a workaround (also the basis for why those files will still play in iTunes): there are a few ways to trick QuickTime into ignoring the invalid track data. For instance, rename the files .3GP and they'll show up just fine.
