I'm assuming your talking is not actual talking but texting or online chat?
I can understand how you might feel you're hearing his voice as you're reading
having looked at some of the other answers as well, maybe you should take a break in communication for a bit
Maybe because you have openings on the sides of your head that allow for his voice to filter into your head so that you can understand what he's saying...and they're called ears.
Not really we all have those weird scary things about us that only we think is weird