I’ve had some problems before with flash not being able to read things that I had downloaded (when I try to import them to the library, that is). Probably because they became corrupt during the xfer or when I unzipped them (I guess). If I were you, I would try to get a different fresh copy of the MP3 you want to use and try it again. When it happend to me, I picked different sound files. Good luck!
Can’t you get the raw wave from your client en encode them yourself? Then you can get real high quilty on low space. Most people don’t know how to compress.