Ok, first let me say, I almost created the name 'Audio Idiot' as my user id. Perhaps I should have; I'm very much a beginner; and in my case, the solution was very simple.
For my problem, it was as simple as clicking 'Save As' and then completely deleting and renaming the file in the file name box. When I clicked 'save as' before, I left the original file name in the file name box and just clicked save in the directory where I wanted the new files stored. That was my problem, because the original file name included the .wma extension of the original file.
Once I actually looked at that and realized what I had done, I deleted the original file name extension (.wma) from that file name box and renamed the file without the extension, it saved it as a mp3 file without any problem.
I'm not sure if that will help in your situation, but good luck with your problem.