- (void)convertToMp3:(MPMediaItem *)mediaItem {
    //get the name of the file.
    NSString *songTitle = [mediaItem valueForProperty: MPMediaItemPropertyTitle];
    //convert MPMediaItem to AVURLAsset.
    AVURLAsset *sset = [AVURLAsset assetWithURL:[mediaItem valueForProperty:MPMediaItemPropertyAssetURL]];
    
    //get the extension of the file.
    NSString *fileType = [[[[sset.URL absoluteString] componentsSeparatedByString:@"?"] objectAtIndex:0] pathExtension];
    
    //init export, here you must set "presentName" argument to "AVAssetExportPresetPassthrough". If not, you will can't export mp3 correct.
    AVAssetExportSession *export = [[AVAssetExportSession alloc] initWithAsset:sset presetName:AVAssetExportPresetPassthrough];
    NSLog(@"export.supportedFileTypes : %@",export.supportedFileTypes);
    //export to mov format.
    export.outputFileType = @"com.apple.quicktime-movie";
    
    export.shouldOptimizeForNetworkUse = YES;
    
    NSString *extension = [export.outputFileType pathExtension];
    NSString *path = [NSHomeDirectory() stringByAppendingFormat:@"/Documents/%@.%@",songTitle, extension];
    
    NSURL *outputURL = [NSURL fileURLWithPath:path];
    export.outputURL = outputURL;
    [export exportAsynchronouslyWithCompletionHandler:^{
        if (export.status == AVAssetExportSessionStatusCompleted){
            //then rename mov format to the original format.
            NSFileManager *manage = [NSFileManager defaultManager];
            NSString *mp3Path = [NSHomeDirectory() stringByAppendingFormat:@"/Documents/%@.%@",songTitle,fileType];
            NSError *error = nil;
            [manage moveItemAtPath:path toPath:mp3Path error:&error];
            NSLog(@"error %@",error);
        }else{
            NSLog(@"%@",export.error);
        }
    }];
}