I had exactly the same thing happen to me on some of my tunes.
It turns out, that the mp3 files ripped from my CDs and encoded at 128kbps with VBR (variable bit rate) refused to play, while the others encoded at 160kbps with CBR (constant bit rate) played flawlessly.
I just re-ripped the CDs that wouldn't play using iTunes at 160kpbs and it solved the issue.
Another thing: if your Mac is set up for automatic software updates, it's possible your iTunes software was automatically updated to a new version. If that's the case, audio streaming is disabled until you start iTunes on your Mac and agree to the license pop-up message that comes with every iTunes software upgrade.
A forum community dedicated to TiVo digital video recorder owners and enthusiasts. Come join the discussion about programming, streaming, content, schedules, home theaters, displays, models, reviews, accessories, classifieds, and more!