As you complete more battles you also unlock more weapons for your units.
On the title screen, choose Password, then put in the following passwords to advance the indicated amount of battles.
(The first five letters are to be put in as the first line, the second 5 as the second line.)
At the game's ending, after the animation is over and when the "Presented by I'MAX" screen is shown, press Up, Up, Down, Down, Left, Right, Left, Right, B, A & Select. If done right, the sound test will appear.