Out of the box, there are a few generic walk-up sound clips. They're the intro.wav files in the /sound directory. You can add more. I'm not aware of anyone sharing those as a mod or anything yet but I wish they would. You can also add walk-up sound files for a specific player in that same directory by having the filename be the player name (e.g. miguel_cabrera.ogg). ".ogg" files are preferred since they're much smaller than .wav files.
Instead of walk-up music, I did a few walk-up sounds as if the player's name were announced over the P.A. system if you're interested:
http://www.ootpdevelopments.com/boar...d.php?t=288969