My 2GHz iMac G5 (now pretty worthtless, compared to the new Intel iMacs) has been giving me some problems with sound playback. This has been occuring only with certain applications, such as Quick Time Player, Windows Media Player, and even iChat would stop playing sounds every now and then.
I got irked the other morning when I wanted to watch his Steveness from San Francisco via the QuickTime stream. So I looked around, and, lo and behold, there is something quite simple that fixes this problem. This Apple KnowledgeBase document explains the problem and solution:
Some audio applications may change your computer's audio settings to a sample rate that is too high for other applications to use. In this situation, system alert sounds still work, and does iTunes, but other applications may have no sound.Now, I don't know which application would have changed my settings, but the fix is quite simple. You simply open /Applications -> Utilities -> Audio MIDI Setup and change the output settings to 44100.0 Hz (the iMac supports up to 96600 Hz).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060111002800782