I'm having this problem, a user posted about it 2 years ago about Pro Tools 10 and apparently they eventually came out with a fix. Again find a work around for Pro Tools 9.0.6???? I'm guessing they've stopped supporting it and won't put out a fix!? Here's the problem below, I copied the original posters post because I'm having the same problem, hope they don't mind.
Sometimes, usually after I import some additional audio data/tracks, some audio tracks just stop playing back.
The meter will move like there is audio, but you won't hear anything. Voice shows assigned. You can solo it...no sound. Sometimes the aux track meters don't move at all.
Only two ways I've found to fix it. One is to save the session, close Pro Tools and reopen Pro Tools, then reopen the session. Then it usually happens again after 10 minutes or so.
The other way is to go to an offending track and just select the output it's assigned to and assign it again (to the same output works). This resets it and it plays again.
Any ideas.....here's the original thread.
http://duc.avid.com/showthread.php?t=311045