Record video with music from iPod Library

Question

I am trying to record video using AVFoundation (AVCaptureOutput + AVAssetWriter) while playing Audio from iPod library instead of the microphone. I took RosyWriter sample code and removed the AudioInput and instead setup AVAssetReader for reading the song in uncompressed format,

NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey,
                                    [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                    [NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
                                    [NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
                                    [NSNumber numberWithBool:NO], AVLinearPCMIsFloatKey,
                                    [NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
                                    nil];



    mAssetReader = [[AVAssetReader alloc] initWithAsset:mAsset error:nil];
    mAssetReaderOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:audioTrack outputSettings:outputSettings];

    if ([mAssetReader canAddOutput:mAssetReaderOutput]) {
        [mAssetReader addOutput:mAssetReaderOutput];
    } 

    [mAssetReader startReading];

and then periodically poll the audio samples. But it's not working correctly. Any advice on how apps like VideoStar or Blux Movie achieve it ?