EDIT CHEER SONG:

Here you can write your own fight song using the simple in-game music writer. For the Team (at the top) you can write a "Chance Theme" (I assume when runners are in scoring position), a "Big chance Theme" (I guess Walk off situations?), and Winning Song.

Then for each individual player you may also write a unique song to be played when he is batting. There are a few fansites for these things in Japan since fight songs are popular there and whenever a player has a new fight song you will eventually find one online. Personally I have never played with this, so am unable to translate all the details.