code: protected void fadeOutCurrentTheme() {
          if (currentTheme != null && currentTheme.isPlaying()) {
              currentTheme.setOnCompletionListener(null);
              stage.addAction(Actions.sequence(
                      VolumeAction.setVolume(currentTheme, currentTheme.getVolume(), 0f, duration, Interpolation.fade),
                      MusicStopAction.stop(currentTheme)));
          }
          currentTheme = null;
      }
docs: Smoothly fades out current theme.