What version? Have you allowed time for the project to cache -green line should be pretty much complete above the thumbnails?
Is the volume slider (at the left of audio tracks) in the normal far-right position?
Has the SOLO button on another track been inadvertently pressed?
Drop a link to the project here, and someone will check it out. It's not difficult, and can be private.
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. It won't be passed on or retained.
Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to share it.