The second those cascading strings hit in “Theme from the Walking Dead,” fans know what’s coming. The musical world of the show is treacherous, but also full of hope: the echoing glitches and rumbling pulses of “Negan” give a sense of the violence that lies in store, but the calming violins of “Sophia” remind us that there’s pathos and comfort to be found, even in the places that seem most hopeless.