Okay, for now I'll assume that you clicked on the Library in the left pane of iTunes, typed in the name of the mp3 file in the search box in the upper right corner of iTunes, and got zippo.

Again assuming, you are using OS 10.x and either iTunes 3 or 4, so we are on the same page.

Go back to the Shared folder, highlight one of your new unplayable mp3 files by clicking once on it, and then hit the Apple (command) key and the i key. It should open a 'get info' window. One of the things it should tell you is where the file is located, and whether it is the original or the alias.

I'll stop there for the moment for your input.