I saved my aiff files to ext. storage. Now how can I retrieve them and convert them to mp3 or wav files?

I was trying to free up space on my internal storage, so when I created files in Garage Band, I thought it would be a great idea to store them in my external hard drive. However, I now wanted to retrieve my files and burn them to a cd. When I dragged and dropped them to iTunes, they showed up in my playlist, but when I tried to burn the playlist, I got a message telling me there were no files in the playlist. Then I looked into putting them back into Garage Band and transferring from there to iTunes. But do not know how to do this, or even if I can do this. Do I have to always convert my aiff files to either wav or mpe files before I store them? Is it a lost cause for me now? Would really appreciate help on this.