If you have Aperture 3 installed, check Software Update. Apple has released a small update for the program, which addresses problems where HD video clips played during slideshows were becoming unsynchronized with the accompanying audio track after performing trimming functions on the clip.
This update Should be available in Software Update, but is also available for download from Apple's support website: http://support.apple.com/kb/DL1013. The update is a small 3.29MB patch.
So far there have been no known issues with this update, but it has only been hour for a few hours. If the features addressed by this updated do not affect you, and especially if you extensively use Aperture, then you may consider holding off on this update until others have tested it out. While Apple recommends it for everyone, it will only benefit those who use HD video clips in slideshows.
As always, be sure to back up your Aperture libraries and other data before installing this update.