public static VolumeAction fadeIn(final Music music, final float duration) {
        return fadeIn(music, duration, Interpolation.fade);
    }
public static VolumeAction fadeIn ( final Music music , final float duration ) { return fadeIn ( music , duration , Interpolation . fade ) ; }
@param music its volume will go from 0 to 1.
@param duration length of the transition.
@return {@link VolumeAction} which should be added to a stage.