The nicest thing of File Typer of course is that you can open them all and apply the same changes to all...
But I would actually test first with just one of the files, see if iTunes plays it (IIRC last version for MacOS 9 was iTunes 2 or 3), if it doesn't work it doesn't matter if you change the file type of them all... If you had MacOS X you wouldn't even need to open them in any app, as long as they have the .mp3 extension and they are recognizable MP3 files you could preview them right in the Finder, either from the File Info window or by selecting the file name in Column view, if the preview doesn't allow you to play it won't play in iTunes anyway (which I've seen happen with some of the MP3s available at MWP webpage).

I'm not that up to date nor a real expert, just have too much memory invested in technical/computer related stuff... can't deny my geekiness.:)
Glad to be of help, and good luck in recovering (or replacing) the files.