To capture the sound of the year 2027 for Deus Ex: Human Revolution, Eidos Montreal recruited a man with hair so glorious the music composer had to be bald to balance it out.
While the graphics folks are busy working on textures and such, the sound folks are busy trying to create the aural identity of a world that doesn't exist. They're trying to come up with sounds we've never heard before. How difficult is that? Try to imagine a sound you've never heard. See? It's nigh impossible, unless you have amazing hair.