I had this issue about 6 weeks ago, never found a solution, and now it has reappeared. I've done many projects since that time with no problems. It appears I'm at risk of losing all of my work here just because of a silly audio glitch.
Issue: My project (wedding edit) was working fine, and then problem seemed to appear out of nowhere. I can't hear any audio from timeline (3-4 tracks of nat sound, music, dialogue). Audio meter is flat as well. Even exporting xml and importing into new project won't bring audio back. Levels are normal audible values for all. Strangely, bars and tone will play audio on timeline without issue, which makes me think this isn't a hardware issue. Problem is persistent across all sequences in project, including new sequences created after initial problem. Issue been replicated on my other Mac.
Footage is Canon DSLR 1080p24 H264 files
Audio is .mp3 at both 44.1 and 48hz
2013 15" MBP Retina 16GB RAM running OSX 10.8.5
2009 27" iMac (2.66 GhZ Intel Core i5) 16 GB RAM running OSX 10.8.5