Alternatives.

He has asked her about the connection between them before. She said she doesn't know exactly what it is or its nature. The only thing she knows is that she is part of the ring, and the ring is part of his Power Shifting Omega ability. 

He can see her and can hear her, but he can't hear her in a loud environment, which suggests that he needs his ears to hear her, not his mind. That was confirmed by the fact that he can't sense her mind at all with his mental ability. 

But if he needs his ears to hear her, does that mean she is communicating with him through physical sound? If it means that, why can't other people hear her when she speaks? 

Then again, his inability to pinpoint her existence or the connection between them might be because of her mental immunity and stealth abilities. So she might actually be connected to him mentally but he can't find out. It is all confusing.