Time Machine can save you. Always have it working. You might need to revert to High Sierra. New OS are always to be suspected until proven, especially if you're in the middle of a project.
Did you upgrade from 10.11 or earlier straight to 10.14?
I'll throw this speculation out there:
The installer would update your disk format from HFS+ to APFS by default.
The new APFS disk format introduces additional lower level container volumes to your disk. Could this be screwing with saved file paths? Maybe Reaper is using absolute paths instead of relative paths somewhere?
Use the search feature when Reaper asks for the missing audio files. Browse to the first one it asks for and it will find the rest by itself if they're in the same directory.
Note the search button in that screen is for finding the original files vs the browse button which is for replacing the original files with new choices.