Reading audio samples via AVAssetReader from http://stackoverflow.com/questions/4972677/reading-audio-samples-via-avassetreader

Reading audio samples via AVAssetReader

CMItemCount numSamplesInBuffer = CMSampleBufferGetNumSamples(buffer);

AudioBufferList audioBufferList;

CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
      buffer,
      NULL,
      &audioBufferList,
      sizeof(audioBufferList),
      NULL,
      NULL,
      kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
      &buffer
    );

for (int bufferCount=0; bufferCount < audioBufferList.mNumberBuffers; bufferCount++) {
  SInt16* samples = (SInt16 *)audioBufferList.mBuffers[bufferCount].mData;
  for (int i=0; i < numSamplesInBuffer; i++) {
    // amplitude for the sample is samples[i], assuming you have linear pcm to start with
  }
}

//Release the buffer when done with the samples 
//(retained by CMSampleBufferGetAudioBufferListWithRetainedblockBuffer)
CFRelease(buffer); 

Have you used this to alter the amplitudes? When I do something like samples[i] *= 0.5 I get very choppy results.

If you want CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer to always work, you need to ask it how big to make the AudioBufferList first, allocate that buffer, and then call it again to get the data. See my example code in this Stackoverflow article. https://stackoverflow.com/questions/27444000/how-do-i-call-cmsamplebuffergetaudiobufferlistwithretainedblockbuffer