Hai forse aggiornato a QT 7.4? Da MacFixit:
Video not playing after QuickTime 7.4 update
Several users are experiencing an issue where various QuickTime media (including movies embedded in Web pages) cannot be played after applying the QuickTime 7.4 update. Some users get audio but no video, while others get video but no audio. One reader writes:
"Upgraded to QT 7.4 (Pro). Am running 10.4.11 on a 2.2 GHZ Core 2 Duo and cannot see QuickTime content on Apple's site, at least. All of the ads, including the new Air ad, either don't display or show the QuickTime logo."
Traditional fixes for similar issues include:
Reinstall QuickTime
Remove ~/Library/com.apple.quicktime.plugin.preferences.plist
Remove QuickTime Plugin.plugin (from /Library/Internet Plugins)
search for conflicting, third-party plugins and remove them
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