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If you think the mother of your child is abusing your daughter, then you'd need to file a motion to modify for the court order, along with a petition for a temporary restraining order. Depending on what you allege as abuse, a Judge will make the decision. You might also want to notify Child Protective Services or the police if the abuse is ongoing.
Yes, you have rights as a father to start the process. But, nothing is guaranteed. The Judge will rely on any testimony of the CPS worker (you may need to subpoena them to have them appear) and your testimony and any evidence you have. So hopefully you have documentation of the abuse (pictures of bruises or injuries, etc), because without evidence or documentation, it will be a case of "he said, she said", and Judges have a hard time making informed decisions when it comes down to that.
If your daughter is 15, you can ask the Judge to speak with her in the Judge's chambers. She's old enough to tell the Judge who she wants to live with and why. Regarding medical information, your existing court order may have put a prohibition on you getting medical information. If it doesn't, take the order to the hospital and tell them that as you are the child's legal father, you have a right to that info.
Well, if stepfather is an adult, he can get his own phone. Presuming he can come and go as he pleases, your daughter could get the MP3 player to him and he could give it to you.
She might also be able to mail it to you if she's ever allowed out of the house on her own (like going to school or something).
If and when it comes to a custody issue, you can ask the Judge to order that the MP3 device is turned over, or have your daughter bring it with her when she comes to court to talk to the Judge.
You would ask for it in the "discovery" process. In cases like this, you're allowed to ask for documentation relating to your case. Your local law library should have a copy of the Discovery Request your state uses. You might want to get it before then, however. If the other parties find out it's important, the recordings might mysteriously disappear.