Do you have a problem where iTunes thinks that some MP3 audio files are QuickTime movie files? If so, such a file will still play just fine in iTunes, but iTunes will not sync it to an iPod, nor share it with other networked computers running iTunes.
The Finder will identify these files as type "MP3 Audio File," but doing a Get Info on it from within iTunes will show it as a "QuickTime movie file" instead. You can see all the files iTunes thinks are movie files by control-clicking on the column headings in the iTunes Music list and selecting Kind (if it's not already visible), then clicking on the Kind column to sort all the "QuickTime movie files" together.
To fix the problem, you'll need to have the command-line version of MPlayer installed. Control-click on one of the songs in iTunes and select "Show in Finder" to see the original file; make a copy of this file to your desktop, then run these Terminal commands:
$ cd ~/Desktop
$ mplayer -dumpaudio -dumpfile MySong-new.mp3 MySong.mp3
Replace MySong.mp3 with the name of the file. This will dump the audio from the file into MySong-new.mp3. You can then delete the original song from iTunes, and drag the new file into iTunes. iTunes will then properly recognize the new file as being MP3 audio, which can be shared and synced.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061030130022224