Some people could read faces or body language - Bran, on the other hand, fancied himself a reader of voices. For some reason the sound of his voice made hers unsteady, which meant she was frightened. He didn't think it was him specifically, since she had no idea who he was, which meant the idea of a disembodied voice speaking was enough to rattle her.
But why? He'd been just a child when he started hearing music no one else could, but it hadn't inspired that shivering sensation that he'd felt in the presence of his grandmother all those years ago.
Perhaps speaking wasn't the right way to approach her. What if he didn't simply talk - what if he sang? He knew she felt some kind of connection to singing, and it might quell her irrational fears, not to mention improve the quality of her voice. Now, however, wasn't the right time. Once he'd returned to the apartment and had some time to think, he would see how well his ventriloquism worked with song and distance.