I don't believe the java AudioStream class supports mp3 natively. According to this link it only supports AIFF, AU and WAV. You'll have to look into using another library to load and play an mp3 file.
FileInputStream fis = new FileInputStream(file);
BufferedInputStream bis = new BufferedInputStream(fis);
player = new Player(bis);
The jar folders you must have: mp3plugin.jar and jlayer.jar (do google)
This site describes how to play a mp3 file in Java