What a testiment of your inner beauty and special spirit, the way you have embraced your daughter while maintaining and supporting healthful memories of her mother.
It appears that up until now, any memory of her mother that is shared with her by the adults in her life is also supported by some fantasy that she needs to have in order to make their relationship real, i.e., daily presents and no discipline. However, as mom's we know the latter isn't quite true, lol. But it's okay, it's a coping mechanism. It's also okay to gently counter with, "Every mother who loves her daughter needs to apply a little discipline and I know without a doubt that one as special as you, blessed with two mommies who love you dearly wouldn't hesitate to give a little time out when needed!" and close it with a wink, a smile and a hug.
The tapes aren't something I would ever expect her to listen to on her own, not at 10 years old or any age that she doesn't request personal listening time to hear. That's just too much to ask of a child; way too emotional a task. In fact, I would only encourage her to listen with the promise that I would be there with her, holding her the entire time. Double fact, I would probably listen before I shared them with her, to be honest; your first job is to protect your daughter (it sounds as if in your heart there are no "steps" in this great family of yours).
I also don't think it is so important that you continue to mention things about her mom, it may imply to her that you have yet to completely embrace her, remember she's 10 and living in a home with two sisters whose biological mother you are. You are doing a great job with keeping her mother's memory alive, but it's not necessary to keep pointing out things, it becomes somewhat unnatural, maybe even uncomfortable, especially in public places where she may only wish to be seen as your daughter, not your step-daughter or the daughter of a deceased parent-she's 10 years old, they think differently, as you know. Allow her to come and inquire about things on her own, and in the interim, just be the mom of three fantastic girls.
You are quite a woman!