I had a composition with several sound effects, and I moved the source files for a couple of the effects. When I went back to AE, those files didn't play. The other sounds did play.
I figured it was a file location problem so I deleted the files from the project, and put them back in. The waveform was still empty and it didn't play, even reimported from the new location.
Thankfully I didn't save since that happened, closed AE, re-opened, and it asked where the files were. I put the location, and it worked normally. In the future, if I do happen to save, how can I fix that?
Additional details can be given as necessary.