Just watching the first episode again today, the music in the intro sequence was quite a cool electronic/heavy metal mix, which could've easily been looped in a fight to some extent. I think they definitely have a grip on what to do with the soundtrack, but I'd wager it was a matter of budget management and lack of time that the sound design is as minimalist as it is, along with some other elements of the show. I think once they have a whole year to prepare for the next series (which I'm banking on), there might be a bit more bravado, when the show has proved itself as a hit again to the BBC.