如何淡化NSSound对象

我为我的Mac写了一个廉价而欢快的声板,我用NSSound播放各种声音:

-(void)play:(NSSound *)soundEffect:(BOOL)stopIfPlaying {
    BOOL wasPlaying = FALSE;

    if([nowPlaying isPlaying])  {
        [nowPlaying stop];
        wasPlaying = TRUE;
    }   

    if(soundEffect != nowPlaying)
    {
        [soundEffect play];
        nowPlaying = soundEffect;
    } else if(soundEffect == nowPlaying && ![nowPlaying isPlaying] && !wasPlaying) {
        [nowPlaying play];
    }
}

而不是仅仅停止它,我希望它能在几秒钟内消失.