Joss talks about the score in this interview with The Skinny.
In another first, Whedon had a hand in composing the film's seedy, noir score, which leaks indelibly into the diegetic in one particularly suave soirée scene. He seems pleased his efforts were noted. "It was really terrifying. I'm not qualified but I felt like this is an opportunity I'm never going to get again. Probably the best moment of the entire thing for me was when we were recording some strings. A cellist asked me, 'Did you meet the actors?' And I realised: Oh my God, she just thinks I'm a composer - that's so legit!" This last part is whispered with a mix of awe and pride.