gistfile1.m

    [audioReader startReading];
    
    // Go back to the beginning of the video to append the audio
    [videoWriter startSessionAtSourceTime:kCMTimeZero];
    
    // Get the next block of date of the audio
    CMSampleBufferRef sample;
    
    // Keep appending the audio as it becomes ready
    while ((sample = [readerOutput copyNextSampleBuffer]))
    {
        // Wait for the audio writer to be ready
        while (![audioWriterInput isReadyForMoreMediaData]) {}
        
        // Append to audio writer with input
        [audioWriterInput appendSampleBuffer:sample];
        
        // Cleanup
        CFRelease(sample);
        sample = NULL;
    }
    
    // Finish the video at the source time
    [videoWriter endSessionAtSourceTime:CMTimeMake(duration-1, 2)];
    
    // Finish 
    [videoWriter finishWriting];