请问怎么在AVPlayer播放实现静音

用下面的代码播放音频：

 -(IBAction)hearAudioMessage:(id)sender
{
ChatObject * obj = [chatArray objectAtIndex:rowIndexInt];
 NSString *str=[NSString stringWithFormat:@"http://www.myserver.com/files/%@",obj.voiceFileString];
    self.palyer=[[AVPlayer alloc] initWithURL:[NSURL URLWithString:str]];
    [self.palyer play];
}

我想在播放期间能静音。

用了下面的代码，但是木有实现：

NSArray *audioTracks = [asset tracksWithMediaType:AVMediaTypeAudio];

NSMutableArray *allAudioParams = [NSMutableArray array];
for (AVAssetTrack *track in audioTracks) {
    AVMutableAudioMixInputParameters *audioInputParams =
    [AVMutableAudioMixInputParameters audioMixInputParameters];
    [audioInputParams setVolume:0.0 atTime:kCMTimeZero];
    [audioInputParams setTrackID:[track trackID]];
    [allAudioParams addObject:audioInputParams];
}
AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
[audioMix setInputParameters:allAudioParams];

AVPlayerItem *playerItem = [AVPlayerItem playerItemWithAsset:asset];
[playerItem setAudioMix:audioMix];

不知道应该怎么能实现呢？