title: iphone-peakPowerForChannel function in AVAudioRecorder doesn't work
body: <p>I am trying to use an AVAudioRecorder instance to record sound, the whole recording progress runs well, except that when I call [recorder peakPowerForChannel:0] trying to get the volume, the return value is always 0, that's pretty odd..</p>
  
  <p>I have checked the recorded audio file, it's totally fine. Then I use AVAudioPlayer to play it, and during playing, i call [player peakPowerForChannel:0], the return value seems to be right and make sense. </p>
  
  <p>How can I get the right value ? I have set that: recoder.meteringEnabled = YES, and called [recorder updateMeters] every time I call [recorder peakPowerForChannel:0].
  The initial of recorder is as follows:</p>
  
  <pre><code>NSMutableDictionary *recordSetting = [[NSMutableDictionary alloc] init];
  //General Audio Format Settings 
  [recordSetting setValue:[NSNumber numberWithInt: 'ima4'] forKey:AVFormatIDKey]; 
  [recordSetting setValue:[NSNumber numberWithFloat:44100.0] forKey:AVSampleRateKey]; 
  [recordSetting setValue:[NSNumber numberWithInt: 2] forKey:AVNumberOfChannelsKey];
  //Linear PCM Format Settings 
  [recordSetting setValue:[NSNumber numberWithInt: 32] forKey:AVLinearPCMBitDepthKey]; 
  [recordSetting setValue:[NSNumber numberWithBool:NO] forKey:AVLinearPCMIsBigEndianKey]; 
  [recordSetting setValue:[NSNumber numberWithBool:NO] forKey:AVLinearPCMIsFloatKey];
  //Encoder Settings 
  [recordSetting setValue:[NSNumber numberWithInt:AVAudioQualityMedium] forKey:AVEncoderAudioQualityKey]; 
  [recordSetting setValue:[NSNumber numberWithInt:96] forKey:AVEncoderBitRateKey]; 
  [recordSetting setValue:[NSNumber numberWithInt:16] forKey:AVEncoderBitDepthHintKey];
  //Sample Rate Conversion Settings 
  [recordSetting setValue:[NSNumber numberWithInt:AVAudioQualityHigh] forKey:AVSampleRateConverterAudioQualityKey];
  recorder = [[AVAudioRecorder alloc] initWithURL:url settings:recordSetting error:nil];
  recorder.meteringEnabled = YES;
  </code></pre>