If you want to hear her voice, and she's the type who says comforting words, I think it's worth telling her what you need. Something like, "I forget when you say certain things (you can tell her they're comforting or not...whatever you can handle saying to her) and it will help me to be able to replay it when I'm struggling."

I often don't remember what was said in my sessions, so I take a little notebook to write things my therapist says that I want to think about during the week. It's it's not for recalling comforting words, though, because my therapist knows I have trouble with that. If ever told her I wanted something positive to record, I have no doubt she'd say sure. After she fainted.