Drawing waveform with AVAssetReader

I reading song from iPod library using assetUrl (in code it named audioUrl) I can play it many ways, I can cut it, I can make some precessing with this but... I really don't understand what I gonna do with this CMSampleBufferRef to get data for drawing waveform! I need info about peak values, how I can get it this (maybe another) way?

AVAssetTrack * songTrack = [audioUrl.tracks objectAtIndex:0];

    AVAssetReaderTrackOutput * output = [[AVAssetReaderTrackOutput alloc] initWithTrack:songTrack outputSettings:nil];

    [reader addOutput:output];

    [output release];



    NSMutableData * fullSongData = [[NSMutableData alloc] init];

    [reader startReading];



    while (reader.status == AVAssetReaderStatusReading){



        AVAssetReaderTrackOutput * trackOutput = 

        (AVAssetReaderTrackOutput *)[reader.outputs objectAtIndex:0];



        CMSampleBufferRef sampleBufferRef = [trackOutput copyNextSampleBuffer];



        if (sampleBufferRef){/* what I gonna do with this? */}

Please help me!

Source: Tips4all