Do you think Mitch would have listened of Morrie hadn't been dying?

Yes, I believe that Mitch would have listened to Morrie even if he hadn't been dying. Mitch had great respect for Morrie, and when he heard about his illness he didn't have to get in contact with him. He did that by choice.

As for your other question, I also blieve that impending death may not make someone's voice penetrate, but it definitely makes people stop and listen.