Do the four DLC actually add extra music to the game - i.e. if you don't have these DLC, you will not hear this music in the game. Is this correct?

I've tried to express that as unambiguously as I can to avoid misinterpretation.

My thinking is that it is all already in the game for everyone and that these DLC only give us the option to buy four extra official soundtrack DLCs.

Nothing wrong with that at all as the music is excellent but folks need to know if the above statement is true.

Hello if it is like ES2, these dlc are tag as Soundtrack so it just allow you to dl these soundtrack and do not add them to the game, 

Check them in deezer, and you will recognize most of them ;) 

So yes for me they already are in the game