After reading this thread and figured out that I was not the only one trying to save an mp3 with this app, I saved my work as an AAC file on the desktop. I then opened it with iTunes and it was automatically added to my "recently added" playlist. From there, I burned it to a disc which gave me the option to save as an MP3.
Hope this might help.
[Version 3.6]
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Sound Studio
Hope this might help.