No sound in Forza 7 [XBOX]

Hi,

I have purchased a disc with Forza 7, after completion of installation (disc and download) the complete sound of the game disapeared during the start up of the game. For the rest everything is fully functional.
Does somebody have any idea? FYI my internet conection is quite slow (11Mb/s) so it has taken a while with some stops, can this be an issue?