Actionscript :: Append MIDI Files Using Bytearray?

I need to append MIDI files: leave header (same for all files) and other meta information, just copy music/score part. I already have MIDI files in appropriate bytearrays, as I guessed I need to use writeBytes, but unfortunately couldn't find which bytes I need to take and copy. Something like this:

var newFileBytes:ByteArray=new ByteArray();
newFileBytes.writeBytes(firstMIDIBytes);
newFileBytes.writeBytes(secondMIDIBytes,8);

Works only partially, file is playable; first part fully and second - only some notes (then player hangs out)

To say truth byteArrays aren't my strong side, as the MIDI file structure.