In Changes, and subsequent stories, we learn Mouse is capable of communicating at a human intellect level. In the subsequent stories, we see that some humans can understand Mouse or other human level intellect canines (for example, Molly and the Alphas in Bombshells). When Harry is in dog form in Changes, Harry can understand Mouse:
I eyed the dog and said, “You can talk. How come I never hear you talk?”
“Because you don’t know how to listen,” my godmother said simply.
Also in the first chapter of Changes, Harry seems to understand Mouse at that same intellectual level:
“I . . .” I paused, trying to get my head around the concept. “I . . . I might have a child.”
Mouse made an uncertain, high-pitched noise.
“Yeah. How do you think I feel?” I stared at the far wall. Then I stood up and reached for my coat. “I . . . think I need a drink,” I said. I nodded, focusing on nothing. “Yeah. Something like this . . . yeah.”
Mouse made a distressed noise and rose.
“Sure,” I told him. “You can come. Hell, maybe you can drive me home or something.”
Harry could have been just guessing at what Mouse meant by interpreting his body language and stuff. I personally think Harry was dissociating enough that he was in a state where he could "hear" Mouse like Molly does.
Do you think Harry capable of understanding Mouse? I doubt he
will ever understand Mouse in the books because, as Jim said, Mouse knows too much. My question is whether he could if he worked on it.