use AVAudioFile

gistfile1.m

// Open the file
NSError *error = nil;
    AVAudioFile *audioFile = [[AVAudioFile alloc]
          initForReading: fileURL
          commonFormat: AVAudioPCMFormatFloat32
          interleaved: NO
          error: &error];
          
// Fetch and print basic info
NSLog(@“File URL:          %@\n”, fileURL.absoluteString);
NSLog(@“File format:       %@\n”, audioFile.fileFormat.description);
NSLog(@“Processing format: %@\n”, audioFile.processingFormat.description);
AVAudioFramePosition fileLength = audioFile.length;
NSLog(@“Length:            %lld frames, %.3f seconds\n”, (longlong)fileLength, fileLength / audioFile.fileFormat.sampleRate);

// Create a buffer to read into
const AVAudioFrameCount kBufferFrameCapacity = 128 * 1024L;
AVAudioPCMBuffer *readBuffer = [[AVAudioPCMBuffer alloc] initWithPCMFormat:
audioFile.processingFormat frameCapacity: kBufferFrameCapacity];



// Read the file a buffer at a time to find the loudest sample

float loudestSample = 0.0f;
AVAudioFramePosition loudestSamplePosition = 0;

while (audioFile.framePosition < fileLength) {
	AVAudioFramePosition readPosition = audioFile.framePosition;
	if (![audioFile readIntoBuffer: readBuffer error: &error]) {
		NSLog(@"failed to read audio file: %@", error);
		return NO; 
	}
	if (readBuffer.frameLength == 0)
		break; // finished
}

// For each channel, check each audio sample
for (AVAudioChannelCount channelIndex = 0;
channelIndex < readBuffer.format.channelCount; ++channelIndex) {
	float *channelData = readBuffer.floatChannelData[channelIndex];
	for (AVAudioFrameCount frameIndex = 0;
	frameIndex < readBuffer.frameLength; ++frameIndex) {
		float sampleAbsLevel = fabs(channelData[frameIndex]);
		if (sampleAbsLevel > loudestSample) {
			loudestSample = sampleAbsLevel;
			loudestSamplePosition = readPosition + frameIndex;
		}
	}
}