According to research highlighted in Your Brain on Art by Susan Magsamen and Ivy Ross, literature and music process differently in the human mind; where text engages the intellect, music acts as a "backdoor" to our emotional centres (the limbic system).
By combining them, we create a unified experience where the story provides the narrative context and the music delivers the subconscious emotional payload.
This combination not only lowers cortisol levels through the physical vibration of sound but also creates a "sympathetic vibration" between the listener and the character, allowing you to feel the story as intensely as you understand it.