Having installed my new Mac Mini, I straight away started up Beeblefish to listen to some BBC radio. Strangely, a RealPlayer error popped up saying "RealPlayer cannot open audio device. Another application may be using it." Over the next week or so, I noticed that I also had no sound from Flash and Windows Media Player (though there was no error message).
I looked all over the net (starting here, of course) and couldn't find any fixes, until I stumbled on this fix by "Tom X" hidden in the depths of the MacFixIt forums:
If you have this problem, open Audio Midi Setup (in /Applications/Utilities) and make sure that the Audio Output Format is set to 44,100Hz or less (you'll find this in the bottom right quadrant of the window). That's it!
From what I understand, this only applies to 10.4, but I no longer have 10.3 or 10.2 on any machines to confirm this.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050817093959692