I don't really know what it's like in Nicolas' mind. This is not one of those moments where Nicolas flees into the text, like a naked child escaping his bath; the character is not exposing himself. He hides, and it's important for the reader to understand this, I don't know what goes on in Nicolas. I can only make guesses at this point.
To sense a history of memories, with no conception of the future, an inverted amnesia. This is what I imagine the lover says:
"It's funny I don't remember what you look like. I know the sound of your voice, at least the sound it makes in my head."