I hope this helps someone...
Most of the issues in this thread (for both Mac & Windows) I believe can be cured by deleting the Preferences files for iTunes. In Final Cut Pro, sometimes the program starts doing incomprehensible things (poor audio, wrong sources, wrong video played with audio or vice versa). The solution is to delete the preferences files for FCP.
I started having pretty severe audio playback quality issues in iTunes for all of my music a few weeks ago & have (not very aggressivly) searched for a solution on the internet with no luck. So, I thought I might try to apply the FCP solution to the problem & it worked perfectly.
The files on mac are found in:
(mac system drive)/Users/(your user name)/Library/Preferences/
The files are:
com.apple.iTunes.plist
com.apple.iTunes.eq.plist
com.apple.iTunesHelper.plist
After you delete these files, iTunes will automatically replace them the next time you launch it & you'll have to put things back the way you had them before, but the audio will play properly.
Hope this helps.