AVFoundation + AssetWriter:生成带有图像和音频的电影

I have to export a movie from my iPhone application which contains UIImage from an NSArray and add some audio files in .caf format that have to start at pre-specified times. Now I have been able to use the AVAssetWriter (after going through many questions and answers on this and other sites) to export the video portion containing the images but cant seem to find a way to add the audio files to complete the movie.I have to export a movie from my iPhone applica