A chunk of my audio files randomly became silenced?

I know similar questions have been asked here before, but I'm noticing some thing a little odd with my specific situation. I have been editing a song for a couple of weeks and everything was perfect until I tried to export. When I listened to the exported version, I noticed a lot of my clips weren't audible. Just little chunks here and there. 
when I went to go look at them on audition, everything still looked the same until I clicked on the tracks that I could no longer here. When I viewed them in waveform, the frequency's that show underneath my clip were mostly gone, besides little .1 second clips. The pics I'm gonna attach will show what I'm talking about better than I can explain it, the circled bit is the only part I can still hear. I didn't touch anything.  I rarely edit in waveform unless it's for tiny 2-3 long clips that need the volume turned down, so I'm not really sure why this happened all of the sudden. I opened up the folder that my project was saved in and attempted to manually drag some of the files from my bounced files folder back onto the track, to replace them, but it didn't work, they were also silent.