"How do I tell a co-worker to turn off her music?"
You don't. You are not your co-worker's boss, and you can only "ask", not "tell".
Some steps you can take:
- Ask her nicely if she wouldn't mind turning her music off or down. Sounds like you have already done this with no effect.
- Ask again, but making it clear that it really bothers you and is interfering with your work (if it is). Make sure you stay polite.
- Ask your boss if he/she could do something about it, making it clear again the effect this is having on you. Sounds like you've already done this with no effect.
- Put up with it. Maybe get some noise cancelling headphones of your own.
Since the music is being played through speakers not headphones it would seem likely that other people in the office have no objection, and maybe even like it. There are workplaces like that. Do not be the person who insists that there be quiet because you like it that way, even if other people prefer the opposite.