You can re-compress any MP3 in Flash itself as long as it will be used within Flash. If you want to dynamically load an external MP3 into Flash, then you'll have to re-compress it in a sound editing application.
Double click the sound file in the Library, uncheck the "Use imported quality" box, and set the other boxes to MP3, Best, and whatever the lowest bit rate you find acceptable. Test the sound quality. Click OK.
Next under File, check the Flash tab on the Publish Settings, to uncheck the "Override Sound Settings" box, if it is checked. It shouldn't be.