I wouldn't even worry about it. Honestly, if you think about it, she is just being herself.
If she were more outgoing, would you try to change that about her? Probably not.
If she loved sports, would you want her to stop playing them simply because other people don't?
We have to let our children be individuals. Obviously, this is the way she is, and when SHE wants to, she knows how to speak up.
Trying to get her to be someone she isn't is only going to make her more defensive and more introverted. She isn't going to suddenly become a confident-in-a-crowd kind of person simply because you try to make her that way. She has to want it for herself.
No one has the right answer for you, really. You know her best...do you really think that pushing her into trying to be more assertive in her responses is going to do the trick? Have you tried just letting her be who she wants to be, and, instead of getting her to do sports, ask her what SHE would like to do...maybe give her a list of 30 things she could do, a few sports thrown in...ice skating lessons, karate, softball, swimming, art classes, dance classes, etc...and let her choose the ones which really interest her...then maybe she will be happier to be the first one to do something.
Good luck to you!