How can I merge 8 part byte arrays of a .mkv file in C#?

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C#5.0

I need to merge 8 byte arrays belong to an MKV file into one byte array,so into original file. When I try this with normal methods such as Buffer.BlockCopy() mkv file does have no sound. I have no problem with mp4,mp3 or some media files I tried. Is it necessary any class file or dll to do this?

EDIT : So I am trying to explain more clearly.
My code can download any file that supports range property dividing filesize into 8 same sized byte array part and using HttpWebRequest.AddRange(int from,int to) method. When all download threads finished I combine all byte array into one byte array in the correct order and then I write it to file with right filename and extension. But as I said just mkv videos have no sound. I have no problem any other file extension.And that's right I don't know much about media file encoding.I hope I can be understandable. Here is my combine code:

byte[] combineDATAS()
{
    byte[] fileDataArray = new byte[contentLength];
    int dstOffset = 0;
    foreach (byte[] array in downloadedDataArrays)
    {
        Buffer.BlockCopy(array, 0, fileDataArray, dstOffset, array.Length);
        dstOffset += array.Length;
    }
    return fileDataArray;
}

void writeDataToFile(byte[] fileDataArray)
{
    Stream fileStr = File.Create(FileName);
    Stream byteStream = new MemoryStream(fileDataArray);

    byte[] buffer = new byte[1024];
    int readBytes = 0;
    int offSet = 0;
    while ((readBytes = byteStream.Read(buffer, 0, buffer.Length)) > 0)
    {
        fileStr.Write(buffer, 0, readBytes);
        offSet += readBytes;
        writingPercentage = (int)((offSet / 1.0 / byteStream.Length) * 100);
    }
    fileStr.Close();
    byteStream.Close();
}