This is a basic on dealing with anyone with earphones in. What state of mind are you in when your listening to music on your mp3 player/ipod, the main reason you've got them in is because you want to drift into your own little world and not be bothered by the people around you.
Now consider what your asking by invading that world, your dragging them out of their own thoughts (whatever they are) they could of been daydreaming, stressing about work/school.. who knows.. But your turning round and demanding that they leave their own thoughts and come into yours. So even if they are polite enough to take them out and wonder what's so important they are met with a "Hi... how are you?"
Unless your offering them something worth-while out of the conversation you've essentially just wasted their time and dragged them out of what could of been a really deep thought process, I know this personally annoys me but from a woman's point of view the instant reaction would be "Seriously? I did all that just so you could try and hit on me that pathetically..." earphones straight back in.
If you really want this situation to work you have to offer 10x more and actually make her glad she took them out, no lame lines or "Hi there, what's your name" and even direct game (You were so beautiful I just had to meet you) will be nullified by the inconvenience you've placed upon her. Try asking something like 'sorry but do you know where the hell we are?' in a jokey sense and then continuing with your conversation and moving it into mid-game or if your determined to be direct just make sure you justify the experience, your going to need to offer a very open and enjoyable conversation very quickly so that she doesn't regret even paying attention to you in the first place. Remember that body language, eye contact and just the emotion you communicate within the first 5 seconds is going to make or break the conversation as they initially are going to want to go back to their music and thoughts as quickly as possible.
Give her an experience, make her reconsider even listening to music in public in the first place if there are people as bold and engaging as your around.
That's it
Slightly cryptic but hopefully you can understand the idea behind it and adapt it to the situation. Best of luck!