I also suddenly got this message this afternoon. I am finishing some projects, and everything went fine. Then suddenly the system got stuck. I even tried with testing files.
This has nothing to do with missing files, even if the message says that.
All the files are in the same directory, and the wave file in the Project window plays 100%
But when exporting is complains that some files are missing.
I was under the impression the wave file in Project is used to transport, so why is SO looking for other files?