Well, in this fanfiction, it could imply that Hermione is so attuned to this person's voice that it has some sort of magical or psychological power over her. It could be a story where the voice represents safety or understanding to Hermione. For example, perhaps she's in a difficult situation and his voice is the one thing that can reach her and bring her out of it. So the fanfic might be about their journey together with the voice as the central connection.
I'm not sure specifically as there could be many different interpretations in fanfiction. It might be about Hermione having a deep emotional or physical reaction just to the sound of a particular character's voice. Maybe it's a story of love at first 'hear' where his voice draws her in and sets off a chain of events that make up the plot of the fanfic.