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He used Spaghetti Western music in a WWII movie, as well as all sorts of other anachronistic music. He always uses anachronistic music, in every movie.
People will enjoy a lot of films more, whether they're his or not, if they get over this idea that what they're seeing and hearing is supposed to always be taken literally. Sometimes it's appropriate to go into a movie expecting literalism. His entire filmography up to this point makes that expectation completely foolish anymore where he's concerned.
This was the first of his films that features any music created specifically for one of his films, a few tracks. He selects music usually from his own record collection. You can think of his films almost as visuals to accompany a mix tape. The music is used for its emotional and tonal qualities and not (necessarily) its literal connection to the film, setting or images taking place.
Tarantino is a collage artist, arranging found objects in an interesting way, generally speaking, and his true talent is writing the glue that connects them all. The rap track bothered my brother but I explained to him, when he thought maybe it would be better to use a Blues piece instead, THAT would actually be more of a mistake. The Blues is closer to the source material but still anachronistic. Its closer proximity to the subject matter while still being incorrect means it's actually more like an error (the line he treads with his use of dynamite and shell casings for cap-n-ball guns).
The contrast created is so great yet it's still so fitting when viewed as a whole. That contradiction...hell, contradiction at so many levels... is one of this film and all of Tarantino's films most interesting qualities. The elements themselves, what they say or mean at face value, none of that is exceptional. It's only when you look at the whole and what all these seemingly contradictory parts mean, or might mean, or might mean to you, or might mean to him, that's what makes his movies worth talking about and viewing repeatedly.