Expander is only going to do you any good if it is an archived file.

It would be helpful if you had mentioned what type of file it is. If it is an mp3, for instance, you can try changing the file type and creator with ResEdit, Norton's FastFind, or iTune Associator (versiontracker). If that doesn't work, SoundJam, if you can find it, can deal with more audio file types than iTunes.

The file could also have been corrupted.