I recently upgraded to iTunes 10 because I was dying to try Ping. Let me rephrase that, I upgraded to iTunes 10 because I hate it but have an iPod, and I do love the Store.
Anyway after upgrading, I found that some mp3s couldn’t be played anymore. Not in iTunes, nor on my iPod. Googling around only led me to two discussions on the Apple forum. But it seems quite some other people are having this problem: the biggest part of your collection will play fine, but some files do absolutely nothing.
So here is a way to fix it: download mp3Validator and let is scan your broken files. In my case, it found several errors in the files, but it could fix them.
After that, no need to reimport the files, iTunes 10 just played them nicely again.
I can’t find any reaction from Apple, which seems logical, but I find it strange that I can’t find any of this in any reviews. Anyway, my guess is that the files were already mucked up (some of my files were ripped years ago), but something changed in iTunes causing it to not play these mp3 files anymore.