Update 10/1/2011: With the latest iTunes 10.5b132, this issue has seemed to be fixed.

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I've been encountering a decoding issue with iTunes 10.5 beta 118 where certain audio files will artifact during playback. Using Finder's preview and Quick Look, the audio plays w/o issue leading me to believe that it isn't a systemwide issue nor is the source file damaged.

Here's a screen capture of the artifacting issue, first played back twice in iTunes 10.5b118, then played in Finder and Quick Look: