All footage has gone out of sync/everything is in slow motion??

Hi, I'm doing a video project with a lot of layering, after I add a new video, I make sure it is synchronized to the music, then I mute and lock the track (to prevent lag).  Yesterday, I went to export my video, so I unlocked and unmuted all the tracks, and almost every single one is severely out of sync/playing in slow motion (this occurs in both the preview and the export).  Here is my export:

https://www.youtube.com/watch?v=lTLGDxMMt94

The slow motion is most noticeable with the clips near the top (look at the one wearing the blue graduation cap and gown), but all of them are playing at wrong speeds/out of sync except the 6 screens that I first imported into the project (at the bottom left, the first three on the bottom two rows).

It seems like the later on I added the clips into the project, the slower they are playing (the clips i added on the first day are playing fine, while the slowest ones are the ones I added most recently)

My system is Windows 8, Intel (R) Core (TM) i7-4700HQ CPU @2.40 GHz.  16gb RAM with NVIDIA GeForce GTX870M graphics.

Please help, I have no idea what to do other than to restart the entire project. 

Daniel