There are two major differences between this two games. #1 Materials have changed, so if you gonna see green car/building or any other model then probably mod is dedicated for the classic version. You can convert cars from Classic to Definitive Edition with this guide. #2 Definitive Edition uses newer Fmod Sound System (FSB5, since Mafia 2 classic uses FSB4), if you can't hear the sounds of vehicles or other sounds then probably fsb files (sound files) are old. You can always extract fsb4 and create FSB5 file with Fmod Engine 2.02.03.