Personally my therapist lets me record all of my sessions now only because I dissociate so bad that I can't remember anything that is said. It sounds like you want a recording to kind of have a crutch or memento of your therapist to listen to and that may be why she only wants you to be able to record a small snippet so that you won't constantly ruminate and use that to rely on rather than learning ways to cope by yourself. I would suggest thinking up a topic that you want to discuss that you feel she's going to respond in a way that you want to hear. Then you can start recording bring up your topic and hopefully she'll respond in the way you want. That will be what you'll be able to listen to. It may be a good idea to bring up the subject of why you feel you need the recording of her voice to get by between sessions that way she's answering that question for you because that's what you're struggling with. Either way she wants you to do the work so whether that's finding a coping mechanism in between sessions on your own or coming up with a topic that you want to discuss in therapy to get you the end result of a recording you want. Good luck to you I know it is always difficult