Hello, try this: select all the offline-audio-clips in the timeline, button right/audio channels.../ and re-configure the source channel "none/none" to "left/right". Works for me...
It´s occurs sometimes when you copy clips betweeen proyects, the configuration of the audio channels dissappear... Yes, weird.
Great news, and a "fix," that I would never have thought of - so I learned something from your thread too. Thanks for that.
Now, if Tatoz' suggestion solved the issue, you might want to look over this article: http://forums.adobe.com/thread/1058744?tstart=0 to better help Adobe support these forums, and to notify others, who come to this thread with similar problems, that you found a solution.