Andy, if you want to get technical about your digital files I'll quickly get out of my depth. However, using Audacity - if you use the "export as wav" or "export as mp3" menu options (under the "file" tab) you will have formats which most people will be able to play on their computers. Upload them as is to your file hosting site.

To burn to CD for your own use your CD burner software will do whatever is necessary to the files to make them CD compatible. For example, using windows media player simply drag the wav or mp3 track to the burn list. Other Catters would be able to do the same after downloading the files.

It's a total guess, but I'm thinking that you may have used Audacity's "save project as" option which saves the files in aup format, only usable by Audacity itself.