The following code will work fine. But I think it only works with .wav format.
public static synchronized void playSound(final String url) {
  new Thread(new Runnable() {
     public void run() {
      try {
        Clip clip = AudioSystem.getClip();
        AudioInputStream inputStream = AudioSystem.getAudioInputStream(
          Main.class.getResourceAsStream("/path/to/sounds/" + url));
        clip.open(inputStream);
        clip.start(); 
      } catch (Exception e) {
        System.err.println(e.getMessage());
      }
    }
  }).start();
}