Howard Shore is a big name in film scores. Not only has he collaborated with David Fincher on numerous films, including Seven, but he also worked with Peter Jackson on The Lord of the Rings, The Hobbit, and even Jackson's insanely long King Kong film. That's a lot of music.
Shore has a wide variety of styles, from the sweeping orchestral scores of Jackson's fantasy films down to writing the original theme song for Saturday Night Live in 1975. Seven lies somewhere between the two. It's urban, but it's also haunting. It's intended to make viewers feel miserable, uncomfortable, and uneasy, like recent episodes of Saturday Night Live. (We kid, we kid.)
Director David Fincher, who got his start doing music videos, likes to use modern bands and create a soundtrack instead of a score. The opening credits feature a remix of Nine Inch Nails' "Closer," i.e., that song with lyrics we can't reprint here, and the closing credits are set against a David Bowie song. These popular music choices overshadow Shore's short score (say that five times fast).