iTunes will not play back select songs ...

I recently downloaded iTunes 9 and it seems to be working all right so far. My only problem is that there are certain songs in my library that will not play. They won't play when clicked on and are skipped over when part of a playlist. These songs have played with previous versions of iTunes, although the problem has persisted since one of the last updates for iTunes 8 (not sure exactly which).

The only thing these songs have in common are that they are singular additions, not ones I've added from the Store or a complete CD. And there are other songs that are singular additions, not from the Store or complete CD, that will still play. As far as I can tell, the vast majority of these are in mp3 format.

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Orignal From: iTunes will not play back select songs ...