With the release of iTunes 8, we have been hearing reports of muddy sounding music or an underwater sound to music since the upgrade.  This happens on both windows and Mac and can be remedied by adjusting the equalizer.  The problem seems to be with the gain settings within iTunes itself.  So to remedy this you can simply turn off the equalizer.  If this does not work, turn the equalizer back on and adjust the gain by lowering the slider and keep it there until Apple fixes the problem in the software.