Apple has released Aperture SlideShow Support Update 1.1, which addresses an HD video clip playback issue. According to Apple, for Mac OS X 10.6 (“Snow Leopard”) users, Aperture 3 slideshows with trimmed HD clips should “now remain properly synced” with their audio.
Apple says this is a recommended update (3.29MB) for Aperture 3 users You can obtain it via the Software Updates component of the Mac OS X Systems Preferences app.