使用AVMutableComposition、AVMutableVideoComposition和AVMutableAudioMix创建新的AVAsset，并将其导出为视频文件。下面是示例代码：

AVURLAsset *originalAsset = [[AVURLAsset alloc] initWithURL:originalURL options:nil];
AVMutableComposition *composition = [AVMutableComposition composition];

AVAssetTrack *videoTrack = [[originalAsset tracksWithMediaType:AVMediaTypeVideo] firstObject];
AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, originalAsset.duration)
                            ofTrack:videoTrack
                             atTime:kCMTimeZero
                              error:nil];

AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetMediumQuality];

AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.renderSize = CGSizeMake(640, 640);
videoComposition.frameDuration = CMTimeMake(1, 30);

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, composition.duration);

AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTrack];
[layerInstruction setTransform:CGAffineTransformMakeTranslation(64, 64) atTime:kCMTimeZero];

instruction.layerInstructions = @[layerInstruction];
videoComposition.instructions = @[instruction];
exportSession.videoComposition = videoComposition;
exportSession.outputFileType = AVFileTypeMPEG4;
exportSession.outputURL = outputFileURL;
[exportSession exportAsynchronouslyWithCompletionHandler:^{
    // Handle export completion or errors
}];

该代码将从原始视频文件中提取视频轨道，并使用AVMutableComposition创建新的AVAsset。AVMutableVideoComposition和AVMutableAudioMix对象用于对导出视频进行剪切和转换。最终，AVAssetExportSession用于将新的AVAsset导出为MPEG-4文件。